WalStateManager does not see a consistent snapshot of WAL state in ZK (race condition)

WalStateManager does multiple ZooKeeper lookups to gather the state of the WALs. This can lead to race conditions, although it is not clear to what extent these matter. This should be investigated further.

`WalStateManager.getAllMarkers()` does one ZK lookup for the children of `/wals`, which gets the list of tservers, then loops through that and gets the WALs under each of those with separate ZK lookups, and then `state()` does additional ZK lookups to get the state of each WAL marker. All of these separate lookups can lead to a very inconsistent view of the WAL states. But... does this matter?

Start by reading WalStateManager.getAllMarkers() and state(), focusing on the ZooKeeper lookups for /wals, tablet servers, WALs, and WAL markers. Determine whether the separate lookups can produce a meaningfully inconsistent WAL view and define what investigation or change is needed.

Written by the indexing model from the issue text.