bridge: support bridge local clean session

This adds the ability to separating bridge clean session settings between
the local and remote endpoints.  Some broker implmentations refuse to allow
non-clean sessions, as they don't support storing messages to be sent to
the connecting broker.  However, this doesn't mean that the local
broker can't be queueing messages to send _out_ to the remote broker.

This PR adds a new bridge connection setting, local_cleansession, that
allows controlling this split. Naming is chosen to be local_ in keeping
with the other local_ settings for bridges.

A test for the six cases of queued/not queued messages in both
directions is added, but v5 testing is currently disabled.  The changes
to support the split are ~independent of protocol version.

Signed-off-by: Karl Palsson <karlp@etactica.com>