CORE-3704: Add new context variables to the SYSTEM namespace.

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/branches/B2_5_Release@58325 65644016-39b1-43b1-bf79-96bc8fe82c15
Backport fix for bug CORE-4134 : Race condition when auto-sweep is started

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/branches/B2_5_Release@58327 65644016-39b1-43b1-bf79-96bc8fe82c15
Backport fix for bug CORE-4135 :  In SS sweep blocks establishment of concurrent attachments

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/branches/B2_5_Release@58328 65644016-39b1-43b1-bf79-96bc8fe82c15