Reuse requester instance if URI was already provisioned

Context here : https://lists.opendaylight.org/pipermail/jsonrpc-dev/2018-September/000201.html

Let's make RequesterSessionImpl to use reference counting.

When Requester was already provided for particular endpoint (URI) just return original instance and increase reference count.

Calling close on returned instance will only decrease reference count.

Once it reach 0, close it finally.