Simplify jsonrpc connection shutdown

Simplify jsonrpc connection shutdown

* eglot.el (eglot--process): Delete.
(eglot-shutdown): Use jsonrpc-shutdown.
(eglot--on-shutdown): Simplify.
(eglot-reconnect): Simplify.
(eglot--connect): Simplify.

* jsonrpc-tests.el (jsonrpc--with-emacsrpc-fixture): Simplify.

* jsonrpc.el (jsonrpc-process-type, jsonrpc-running-p)
(jsonrpc-shutdown): New methods.

* eglot-tests.el (auto-reconnect): Use jsonrpc--process.
(eglot--call-with-dirs-and-files): Use jsonrpc-running-p.