CreateEntry starts with a FindEntry to load the current entry. A conditional
CreateEntry already fetched that entry to evaluate the precondition under the
per-path lock, so the create repeated the lookup.
Add an existing *Entry parameter: when non-nil it is used as the current entry
and the internal lookup is skipped; nil keeps the lookup. The gRPC CreateEntry
handler passes the entry it fetched for the precondition, removing the redundant
read while the lock is held. All other callers pass nil.