Jrpc-connect is now passed a generic dispatching function

Jrpc-connect is now passed a generic dispatching function

* eglot.el (eglot--dispatch): New helper.
(eglot--connect): Use it.

* jrpc.el (jrpc--dispatcher, jrpc--request-continuations)
(jrpc--server-request-ids): New process-local var.
(jrpc--pending-continuations, jrpc--method-prefix): Remove.
(jrpc-connect): Take DISPATCHER instead of PREFIX.
(jrpc--process-receive): Use proc's dispatcher.
(jrpc--process-send): Make private.
(jrpc-forget-pending-continuations, jrpc-async-request)
(jrpc-reply, jrpc-notify): Use new function names.