Resolve LATEST_TIMESTAMP to current server time before scanning for ACLs

Storing values with timestamps in the future is probably bad practice and can lead to surprises. If cells with timestamps in the future have ACLs, permissions from those ACLs will incorrectly be considered for authorizing the pending mutation. For sure that will be surprising.

We should be able to avoid this case by resolving LATEST_TIMESTAMP to the current server time when creating the internal scanner for finding ACLs in the covered cell set.

Documenting a todo item from a discussion between anoop.hbase and myself.