bc_http: call reset_stream_state on new connection

Previously we called this inside the error handlers, but it makes more
sense (and is less error prone) to reset the state once we actually get
a new stream.

Change-Id: I931446b246e296d50e13bab1d62e38feb36508ce
Reviewed-on: http://review.couchbase.org/37857
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>