[Membership] Preserve latest IAmAliveTime across updates

[ReubenBond]

In subsequent PRs, we leverage the IAmAliveTime value for more behavior related to disaster recovery scenarios. Each silo's IAmAliveTime value is updated on the membership table directly by that silo and it does not bump the membership table's version. Therefore, we cannot use IAmAliveTime in any algorithm which requires consistent membership views (eg, monitoring graph construction, directory membership). IAmAliveTime updates can be occasionally ignored or regressed by membership snapshot updates depending on when the snapshot was captured. That is undesirable but incrementing the membership version for every IAmAliveTiem would greatly increase write contention on the table and cause unnecessary churn for algorithms based on membership version, such as directory membership.

This PR improves upon the situation by locally preserving the latest known IAmAliveTime values for each silo across membership snapshots. It also allows updating snapshots without incrementing the snapshot version by determining if a snapshot is a logical successor to a previous snapshot (higher version or equal version with at least one greater IAmAliveTime value).

The goal is to decrease the chance of incorrectly treating a silo as "stale" (having missed multiple IAmAliveTime updates).

Microsoft Reviewers: Open in CodeFlow