storage/log: Generalize log consuming and position acknowledgment

A follow-up of !7424 (merged)

In the above MR, log management responsibility was extracted out of the TransactionManager. We introduced a new position-tracking system as an extension of the existing consumer acknowledgment system. Due to the massive change size of the MR, we settled down with an acceptable acknowledgment interface.

The LogManager is aware of different types of tracking positions while it should not to. The consumers need to call specialized position acknowledgment methods. This interface is not universal enough given the context that there might be more consumers in the future. This issue tracks the effort to improve the consuming interface.