Revert "Revert "SERVER-15870 Prune command stats for commands that haven't run""

This reverts commit 3e31d5fab0a5ab48378a04b01ae045997af1d41a.
SERVER-15870 fix up some tests depending on unpruned serverStatus
SERVER-15889 Update list of targetable OS X versions
SERVER-14747 SERVER-14753 Add validation tests for memory leak fixes
SERVER-15888: Ensure MODE_X on implicit DB creation

Also adds new API to DBLock to allow relocking in different mode.
Merge remote-tracking branch 'root/master'