jsonrpc-server: split monitors into database back end and JSON-RPC front

jsonrpc-server: split monitors into database back end and JSON-RPC front
end

jsonrpc-server.c has two main functions. One deals with handling the
jsonrpc connections, the other deals with monitoring the database.

Currently, each jsonrpc connections has its own set of DB monitors.
This can be wasteful if a number of connections shares the same
monitors.

This patch, and a few following refactoring patches attempts to
split the jsonrpc handling front end off the main monitoring
functions within jsonrpc.c.

This patch changes the monitoring functions and data structures from
'ovsdb_jsonrpc_monitor_xxx' into 'ovsdb_monitor_xxx'

This and the following patches move the ovsdb_monitor backend functions
into their own file.

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