Timeouts for idle database sessions.
  
Author:
  Vlad Khorsun <hvlad@users.sf.net>


Description:

  The feature allows to automatically close user connection after period of inactivity.
It could be used by database administrators to forcibly close old inactive connections
and free resources it occupies. Application and tools developers also could find it as 
easy replacement of self-made control for the connection life time. 
  It is recommended (but not required) to set idle timeout to reasonable big value, such 
as few hours. By default it is not enabled.

  The feature works as below
- when user API call leaves engine, special idle timer assotiated with current connection 
  is started
- when user API call enters engine, idle timer is stopped
- when idle time is fired engine immediately closes the connection in the same way as 
  with asyncronous connection cancellation:
  - all active statements and cursors are closed
  - all active transactions are rolled back
  - network connection is not closed at this moment. It allows client application to get 
    exact error code on next API call. Network connection will be closed by the server 
	side after error is reported or due to network timeout if client side disconnects. 
- idle session timeout could be set:
  - at database level, by setting value in firebird.conf (or databases.conf) by database 
    administrator
    scope - all user connections, except of system connections (garbage collector, cache 
		writer, etc)
	units - minutes
  - at connection level, using API and\or new SQL statement (see below)
    scope - given connection
	units - up to seconds
- effective value of idle timeout is evaluated every time user API call leaves the engine
  as:
  - if not set at connection level, look at database level
  - in any case can't be greater than value set at database level 
  i.e. value of idle timeout could be overriden by application developer at given 
  connection but it can't relax limit set by DBA (in config)
- zero timeout means no timeout, i.e. idle timer will not start
- while idle timeout is set in seconds at API level, we can't promise absolute precision. 
  With high load it could be less precise. The only guarantee is that timeout will not 
  fire before specified moment.
- if connection was cancelled, next user API call returns error isc_att_shutdown with 
  secondary error code specifying exact reason:
  isc_att_shut_killed:		Killed by database administrator
  isc_att_shut_idle:		Idle timeout expired
  isc_att_shut_db_down:		Database is shutdown
  isc_att_shut_engine:		Engine is shutdown

  
  Support at configuration level (firebird.conf and\or databases.conf)
 
  New setting "ConnectionIdleTimeout": set number of minutes after which idle connection 
will be disconnected by the engine. Zero means no timeout is set.
Per-database configurable. Type: integer. Default value is 0.

  
  Support at API level
  
- get\set idle connection timeout, seconds
interface Attachment 
	uint getIdleTimeout(Status status);
	void setIdleTimeout(Status status, uint timeOut);

- get idle connection timeout at config and\or connection level is possible
  using isc_database_info() API with new info tags:
  - fb_info_ses_idle_timeout_db			value set at config level
  - fb_info_ses_idle_timeout_att		value set at given connection level
  - fb_info_ses_idle_timeout_run		actual timeout value for given connection
										evaluated considering values set at config and 
										connection levels, see "effective value of idle 
										timeout" above

Remote client implementation notes:
  - Attachment::setIdleTimeout() issued "SET SESSION IDLE TIMEOUT" SQL statement
  - Attachment::getIdleTimeout() calls isc_database_info() with 
    fb_info_ses_idle_timeout_att tag 

If remote server doesn't support idle connection timeouts (protocol version less than 16):
  - Attachment::setIdleTimeout() will return isc_wish_list error
  - Attachment::getIdleTimeout() will return zero and set isc_wish_list error 
  - isc_database_info() will return isc_info_error tag in info buffer (as usual).


  Support in SQL

- New SQL statement allows to set idle connection timeout at connection level:

	SET SESSION IDLE TIMEOUT <value> [HOUR | MINUTE | SECOND]

if timepart is not set, default is MINUTE.
This statement could run outside of transaction control and immediately effective.

- Context variables

  Context 'SYSTEM' have new variable: 'SESSION_IDLE_TIMEOUT'. It contains current value 
of idle connection timeout that was set at connection level, or zero, if timeout was not
set.

- Monitoring tables

  MON$ATTACHMENTS
    MON$IDLE_TIMEOUT		Connection level idle timeout
	MON$IDLE_TIMER			Idle timer expiration time

  MON$IDLE_TIMEOUT contains timeout value set at connection level, in seconds. Zero, if 
	timeout is not set.

  MON$IDLE_TIMER contains NULL value if idle timeout was not set or if timer is not 
	running.