Guard against null TimeoutsHolder in NettyConnectListener.onSuccess

[hyperxpro]

Motivation

NettyConnectListener.onSuccess NPEs on timeoutsHolder.setResolvedRemoteAddress(...) when a request timeout fires concurrently with a successful connect: abort() calls cancelTimeouts() (nulls the holder) before setting isDone, and per JMM the IO thread can observe holder==null while isDone==0. PR #2127 only guarded the remoteAddress parameter, not the holder.

Modification

• Add null guard on timeoutsHolder in onSuccess — close the channel and bail out if the holder was nulled out concurrently.
• Broaden futureIsAlreadyCancelled → futureIsAlreadyCompleted (isCancelled() → isDone()) so aborted futures don't slip past the early-out (abort()/done() set isDone, not isCancelled).
• Release the partition-key lock on both early-exit paths.
• Add NettyConnectListenerTest reproducing the exact NPE from the bug report.

Fixes
#2172

[hyperxpro]

@jaredstehler PTAL