Attempted to fix CORE-4235: Deadlock is possible while accessing the monitoring tables under concurrent load.
Refactored the list of reference counted attachments to be useful in other cases.
Some minor adjustments in the monitoring code.

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/trunk@58660 65644016-39b1-43b1-bf79-96bc8fe82c15