The different read sources establish its readAt timestamp in different ways in the RecoveryUnit.

kProvided and kLastApplied sets the timestamp when setTimestampReadSource is called.

kNoOverlap, kAllDurableSnapshot, kMajorityCommitted opens a snapshot if not already opened and sets the timestamp when calling getPointInTimeReadTimestamp.

The different behavior around when a timestamp is set and which calls will open a snapshot makes it difficult to use the API in a correct way that will work in a consistent way for the different read sources. We should try and unify the behavior.