Cleanup ReconnectPromise a bit

Cleanup ReconnectPromise a bit

ClosedChannelHandler is really a stateless object used by
ReconnectPromise wiring. Acknowledge that fact by removing the class in
favor of a field initialized by an anonoymous inner class.

While we are here, also document locking rules and generally tighten
things up, eliminating a SpotBugs suppression in the process.

Change-Id: I6abc0ddf296dd9e2f5ea6de5a709003cf14edc7a
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 82f3e83d6e777a5e76b8b6bca5a8c2646a4b82be)