ovsdb-monitor: add ovsdb_monitor_changes

ovsdb-monitor: add ovsdb_monitor_changes

Currently, each monitor table contains a single hmap 'changes' to
track updates. This patch introduces a new data structure
'ovsdb_monitor_changes' that stores the updates 'rows' tagged by
its first commit transaction id. Each 'ovsdb_monitor_changes' is
refenece counted allowing multiple jsonrpc_monitors to share them.

The next patch will allow each ovsdb monitor table to store a list
of 'ovsdb_monitor_changes'. This patch stores only one, same as
before.

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