Monitor dispatcher connection when receiving from TCP interconnect
Monitor dispatcher connection when receiving from TCP interconnect

This is mainly to resolve slow response to sequence requests under
TCP interconnect, sequence requests are sent through libpqs from
QEs to QD (we call them dispatcher connections). In the past, under
TCP interconnect, QD checked the events on dispatcher connections
every 2 seconds, obviously it's inefficient.

Under UDPIFC mode, QD also monitors the dispatcher connections when
receving tuples from QEs so QD can process sequence requests in
time, this commit applies the same logic to the TCP interconnect.

Reviewed-by: Hao Wu <530637259d3f9d3c2e7d9cd9677d335a7c7e78ee@gmail.com>
Reviewed-by: Ning Yu <1ef20c7bf83cc456c790d6afa040b63811531869@pivotal.io>

50wu/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,50wu/gpdb,xinzweb/gpdb,50wu/gpdb,greenplum-db/gpdb,adam8157/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,50wu/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,adam8157/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,adam8157/gpdb,xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,jmcatamney/gpdb,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,lisakowen/gpdb,jmcatamney/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,adam8157/gpdb,jmcatamney/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb