kMajorityCommitted ReadSource should not have special treatment in RecoveryUnit

kMajorityCommitted is special in that it requires a call to obtainMajorityCommittedSnapshot(), which just returns a Status and sets _majorityCommittedSnapshot. That snapshot (now Timestamp) is a lie, however, because the variable is overwritten when a transaction is actually opened. There can't be any callers that depend on the getPointInTimeReadTimestamp() being accurate, because the Timestamp can advance between that call and opening a transaction.

This confuses the RecoveryUnit API unnecessarily and is an exception to how every other ReadSource operates.

I propose the following improvements:

• Replace obtainMajorityCommittedSnapshot() with a const function that only returns a Status. Name it majorityCommittedSnapshotAvailable
• Remove _majorityCommittedSnapshot and use _readAtTimestamp like every other ReadSource.