Add switch to control in guardian timeout before killing firebird server process

[AlexPeshkoff]

Currently FB guardian waits for 3 seconds after notifying server that it's time to shutdown and kills it. That's bad strategy for big cache - server has not enough time to flash it to the disk. Timeout value should be set by guardian's switch, default should be increased.

[AlexPeshkoff]

In FB 3 & 4 default timeout is left 3 seconds, increased to 30 - only in FB5 and higher.