Jsonrpc: rework implementation of continuations

Jsonrpc: rework implementation of continuations

Preparatory work for fix of bug#67945

* lisp/jsonrpc.el (jsonrpc-connection): Change slots.
(jsonrpc--remove): New helper
(jsonrpc-forget-pending-continuations)
(jsonrpc-connection-receive)
(jsonrpc-request)
(jsonrpc--process-sentinel)
(jsonrpc--async-request-1)
(jsonrpc--async-request-1): Rework.
(jsonrpc-continuation-count): New convenience helper.

* lisp/progmodes/eglot.el (eglot--mode-line-format): Stop using
jsonrpc--request-continuations.