ovsdb: Add raft memory usage to memory report.

ovsdb: Add raft memory usage to memory report.

Memory reports could be found in logs or by calling 'memory/show'
appctl command.  For ovsdb-server it includes information about db
cells, monitor connections with their backlog size, etc.  But it
doesn't contain any information about memory consumed by raft.
Backlogs of raft connections could be insanely large because of
snapshot installation requests that simply contains the whole database.
In not that healthy clusters where one of ovsdb servers is not able to
timely handle all the incoming raft traffic, backlog on a sender's side
could cause significant memory consumption issues.

Adding new 'raft-connections' and 'raft-backlog' counters to the
memory report to better track such conditions.

Acked-by: Han Zhou <hzhou@ovn.org>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>