AMQP-358: Cache CachedConnection's first channel

JIRA: https://jira.springsource.org/browse/AMQP-358

Previously, each `Connection#createChannel` call of Cached Connection for
`CacheMode.CONNECTION` caused the creation of new `Channel`.

Provide the hook to cache the single `Channel` for each Cached Connection
and infrastructure logic to get deal with it's lifecycle.