Fixed bug, reported privately by Dmitry

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/trunk@58790 65644016-39b1-43b1-bf79-96bc8fe82c15
Prevent races by allowing the worker thread to process op_exit / op_disconnect packets only after the listener had synchronously called cancel_operation(). This fixes CORE-4268: Server may crash while disconnecting from a database.

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/trunk@58791 65644016-39b1-43b1-bf79-96bc8fe82c15