s3: route non-versioned object PUT and DELETE off the distributed lock
A non-versioned, non-object-lock object write now goes straight to the key's
owner filer as a single-mutation ObjectTransaction, which serializes it with the
owner's per-path lock and evaluates the precondition, instead of taking a
cluster-wide lock. PUT and DELETE use the object's full path as the lock key, so
a concurrent create and delete of the same key serialize against each other.
The fast path is taken only when the precondition reduces to clauses the filer
can evaluate (existence and a single strong-ETag match); time-based conditions,
ETag lists, weak ETags, post-create hooks, and an unknown owner fall back to the
lock. A routed mutation error other than a failed precondition also falls back,
so the lock path stays the authority for the cases it alone covers.
PrimaryForKey returns "" until the ring view arrives, keeping writes on the lock
until routing is known.
