potential memory leak in JsonRpcEndpoint

Summary:
If the same OVSDB client repeatedly makes requests to read the OVSDB configuration, the methodContext hashmap keeps growing.

Steps to reproduce:

• add multiple ports/interfaces (>30) to OVSDB (using ovs-vsctl) so that the increase in memory consumption is easy to detect.
• using a client, make requests to read the configuration in OVSDB repeatedly.
• observe the memory growth of this test program using a tool like jstack.

Some questions:

• Is there a recommendation that the same OVSDB client should not be used for multiple requests?
• Is there a way to reinitialize JsonRpcEndpoint for the same OVSDB client?