SERVER-14668: Replace uses of DBWrite lock with DBLock

This reverts commit 8e83e72512fcb8eb8f06987927766c0b77cea23e.

This reinstates commit ae333bc94a7d89d3220dcae9579fcaf68aa2e290
and commit 962f959a09b63aa0482d7e0c9bad89363d1e1194, and fixes
three cases where the wrong database name was locked.
Update README

Updated description of MMS.
Added MongoDB University link.
Merge remote-tracking branch 'upstream/master'