Instead of maintaining a vector of the sentinel values in memory, OplogBufferCollection should store sentinels on disk. The _id field format would be changed from:

_id: <Timestamp>

to:

_id: { ts: <Timestamp>, sentinel: <i> }

Where i is the number of sentinels with the same timestamp <Timestamp>.

This change would improve readability and make it easier to implement batched reads from the collection in SERVER-26191