Provide new interface for getting new OpTimes for vector insert from the optime generator

Currently, the optime generator is only accessed via logOp() and logInsertOps() called via an opObserver.
There is a need for being able to obtain (and reserve) optimes prior to opObservers being run; the work for this ticket will be to thread through an interface for obtaining a new block of OpTimes and providing logInsertOps() with these times.

One such need is for transactions that write multiple oplog entries, such as vectored insert.