Jsonrpc: deal with nested synchronous jsonrpc-request

Jsonrpc: deal with nested synchronous jsonrpc-request

See bug#67945

* lisp/jsonrpc.el (jsonrpc-connection): Add -sync-request-alist
(jsonrpc-connection-receive): Rework.
(jsonrpc-request): Rework.  Pass SYNC-REQUEST to
jsonrpc-async-request-1.
(jsonrpc--process-sentinel): Simplify.
(jsonrpc--schedule): New helper.
(jsonrpc--continue): New helper.
(jsonrpc--async-request-1): Rework.
(jsonrpc--process-sentinel): Also cancel deferred action timers.
(Version): Bump to 1.0.21

(cherry picked from commit 02b99db661861905162a6638349936e784df3189)