CreateEntry is a FindEntry-then-write with no lock, so concurrent creates to the
same path race: OExcl can admit two creators, and a conditional check-then-act
has no atomicity. Add a per-path exclusive lock (util.LockTable, which evicts
idle keys so it stays bounded) on the FilerServer and take it in CreateEntry, so
the existence check and the write are atomic on this filer.
This is the local serialization point that lets callers route a key's writes to
its owner filer and drop the distributed lock for that key. AppendToEntry keeps
its distributed lock for now; it can move to the per-path lock once its callers
route to the owner.
