Add write concern option to not wait for snapshot on majority writes

Once ReadConcernMajority support is enabled snapshots are created periodically. But when WriteConcern.Majority is used, it must wait for a snapshot so that ReadConcern.Majority can then read that write.

There are cases when one might want to do majority writes but not wait for a snapshot – there is no need to read your own writes using ReadConcern.Majority basically.

// example

coll.insert({}, {writeConcern:{w:"majority", waitForSnapshot:false}});