fixes #710 remove $snapshot in the core server, did not remove it from
cpp/js client libraries for compatibility reasons
fixes #712 remove showDiskLoc from the ParsedQuery class
fixes #709 fix up the condition for fsync on getLastError
fixes #707 no need to RetryWithWriteLock in oplog processing code
fixes #714 Add constness to the oplog API and access the current
client's TxnContext via cc().txn(), not by parameter.
refs #708 remove Cursor::capped()
refs #708 remove QueryPlanSelectionPolicy::idElseNatural
refs #708 remove DocumentSourceFilter, dead code since 2011
refs #708 remove ClientCursor::assertNoCursors
refs #708 remove QueryPlan::newReverseCursor()
refs #708 remove a client context constructor previously used by
ReadContext