ovsdb-monitor: allow multiple jsonrpc monitors to share a single ovsdb

ovsdb-monitor: allow multiple jsonrpc monitors to share a single ovsdb
monitor

Store ovsdb monitor in a global hmap. If a newly created ovsdb monitor
object monitors the same tables and columns as an existing one, the
existing monitor will be reused.

With this patch, jsonrpc monitor and ovsdb monitor now have N:1 mapping.
The goals are to:
1) Reduce the cost of maintaining duplicated monitors.
2) Allow for create Json cache for the same updates. Json cache will be
introduced in the following patch.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>