Factor the connection-backed write handle pool out of the smb backend
into a generic lib/filepool.Pool[T] with its own tests, so it can be
reused by other backends that implement fs.OpenWriterAter over a
connection pool.
The smb backend keeps its behaviour, opening and releasing handles
through small closures passed to the pool.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Splainte <r.wycke@hotmail.fr>
