ovsdb-monitor: stores jsonrpc-monitor in a linked-list

ovsdb-monitor: stores jsonrpc-monitor in a linked-list

Currently, each ovsdb-monitor points to a single jsonrpc_monitor object.
This means there is 1:1 relationship between them.

In case multiple jsonrpc-monitors need to monitor the same tables and
the columns within them, then can share a single ovsdb-monitor, so the
updates only needs to be maintained once.

This patch, with a few following patches,  will allow for N:1 mapping
between jsonrpc-monitor and ovsdb-monitor.

Maintaining jsonrpc-monitor pointers in a linked-list is essential
in allowing N:1 mapping. The ovsdb-monitor life cycle
is now reference counted. An empty list means zero references.

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