SERVER-1423 Global/Flush S acquisitions should set different policy

Global S-mode should set a compatible-first policy in order to allow reads
to happen even if there are writers which might be blocked.
SERVER-16067 Split new lock acquisition and conversion into two methods

This makes it clear where we do acquisitions and conversions and we can do
less work and decision making while holding the Lock Manager's bucket
mutex.
SERVER-16138: make sure inserts to oplogs update highest value seen
SERVER-16136 fix drop index errmsg
Merge remote-tracking branch 'upstream/master'