MB-29973: Don't reset connection priority to medium after DCP StreamRequest
MB-29973: Don't reset connection priority to medium after DCP StreamRequest

During investigation of a performance regression (MB-29675), it was
noted that DCP replication queue wasn't stable - it continued to grow
until the front-end workload finished; at which point it drained
relatively quickly.

Looking at the connection stats, while the replication DCP connections
have their priority listed as "high"; the actual memcached-level
connection is set to "Medium":

    ‡ ../install/bin/mcstat -h localhost:12002 -u Administrator -P asdasd connections | \
        cut -d ' ' -f 2- | \
        jq -c 'select(.dcp == true) | {peer: .peername, priority: .priority, max_reqs_per_event: .max_reqs_per_event, total_send: .total_send, total_recv: .total_recv}'
    {"peer":"127.0.0.1:33454","priority":"Medium","max_reqs_per_event":20,"total_send":56534,"total_recv":33358138}
    {"peer":"127.0.0.1:59406","priority":"Medium","max_reqs_per_event":5,"total_send":48274133,"total_recv":42346}

    First connection is DCP consumer (receiver), second is DCP producer
    sender).

As such, the DCP replication streams are only permitted 5 request per
libevent read event (Medium priority), which is 10x fewer than the
expected number at High priority. Given that the default (unspecified)
priority allow 20 events, this means that DCP replication streams are
actually de-prioritised compared to normal client connections.

Issue is that on successful completion of a DCP Stream Request the
connection priority is unconditionally set to Medium. Fix by removing
this (and just relying on the default set in ep-engine Producer
constructor).

Change-Id: Ib15132786498666030a939589966386a4a65ab03
Reviewed-on: http://review.couchbase.org/95186
Reviewed-by: Tim Bradgate <f1a26ee65f021cb7d82b83dfb09abf144d3eb64f@couchbase.com>
Reviewed-by: David Haikney <429c6ca43f70a1a1104bd68d6b2835bd23bff953@couchbase.com>
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine