Allow instantiation of more efficient RpcServiceInvokers

Allow instantiation of more efficient RpcServiceInvokers

Current invoker derived from binding class has the downside of
performing localName->methodName conversion on each invocation, which
costs us in the lookup path, as deriving the binding method name is a
costly operation.

Add a new static factory method for creating a RpcServiceInvoker
instance base on a QName->Method map. Provide two additional
implementations: a generic one, which performs a direct lookup in the
map and a specialized one, which performs lookup only on the localName
part of the qname. The latter is used after we have made sure all QNames
come from the same QNameModule.

Change-Id: I6a4b33f0487838b3ae6c15470823e4bc01114aa5
Signed-off-by: Robert Varga <rovarga@cisco.com>