MB-11461 Use a single upr message for gathering vbucket-seqno stats

The view engine has to get vbucket sequence numbers for all partitions
whenever a query is processed. The time taken for requesting the stats
and collecting partition sequence numbers affect the query latency
during stale=update_after and stale=false queries. Currently we are
requesting one message per partition to the EP-Engine. That means, for
1024 partitions, we will need to process 1024 upr messages.

Added support for sending single upr vbucket-seqno stats message and
parse interested partition seqnos. This should reduce the network
noise caused by stats requests and improve query latency.

Change-Id: I602e11923fd26980e1f08dab836b4dfa19a02eb3
MB-11505 Use a separate upr connection for stats

Currently, stats and upr stream requests are interleaved
in a single tcp connection. Stats is a frequent call that
is used by view_group for obtaining current partition
sequence numbers from kv server. Stats should not get
blocked by any other stream requests. The time taken
by stats call affects the query latencies.

Change-Id: I961e27b29608f13feb078f681de105a7a0120451
MB-11505 Provide partition seqnos as part of internal query request

All stale=false and stale=update_after group requests will need to
trigger index updater. Before starting the index updater, it has
to calculate the partitions current high sequence number. It is
a time expensive operation performed within the view group process.
It will cause contention in the view group and all processes who
wants to access the view group will be queued and blocked.

Moved partitions seqnos calculation as responsibility of the process
who is requesting the group information from the view group.

Change-Id: I5ac863297656e817bc9fd23ef996f8d6389c2ca8