Cut fixed per-request costs out of the reshard control plane

[copybara-service]

Cut fixed per-request costs out of the reshard control plane

Every Stage-3 coordination paid four avoidable fixed costs on its framed
RPCs (coordinate, GET_METADATA, receiver arm):

1. The 4-byte length prefix and the body went out as two separate send()
   calls with Nagle enabled, on requests and responses alike, exposing
   every hop to the delayed-ACK stall (tens of ms on small RPCs).
2. The destination controller was asked for every registered unit's full
   pool manifest on every request, although work units register once per
   engine lifetime.
3. Client sockets carried no keepalive, so a black-holed peer was only
   detected at the full receive timeout.
4. The framed server never reaped its per-connection threads: one
   std::thread handle and stack per request, held until shutdown.

Changes:

• framed_rpc: single-buffer framing on both directions; TCP_NODELAY on
  client and accepted sockets; SO_KEEPALIVE plus TCP_USER_TIMEOUT bounded
  by the call's I/O timeout on client sockets; the accept loop joins
  finished connection threads.
• reshard_coordinator: destination metadata is cached per controller
  address. Staleness (engine replacement) surfaces as a plan-build or
  receiver-arm failure. A failed attempt that used the cache drops the
  entry; while no receiver has acknowledged its arm the attempt is
  side-effect-free beyond the abandoned claim and is replayed once on
  fresh metadata. Once any receiver has acknowledged, the failure is
  returned as is and the next request re-queries.

Validation: reshard package builds and reshard_service_test passes in the
OSS bazel build (ml-build container, clang-18), including the new
RemoteMetadataCachedAndRefreshedOnStaleFailure test (cache hit on the
second request, exactly one refetch after a fingerprint-mismatch replay)
and PartialReceiverArmFailureDropsCacheWithoutReplay test (one of two
receivers refuses its arm: no replay, the cache entry is dropped, and the
next request re-queries).

GitHub: #808