Implement asynchronous server connection

Implement asynchronous server connection

A new defcustom eglot-sync-connect controls this feature.  If it is t,
eglot should behave like previously, waiting synchronously for a
connection to be established, with the exception that there is now a
non-nil timeout set to eglot-connect-timeout, which defaults to 30
seconds.

eglot-connect is now considerably more complicated as it replicates
most of the work that jsonrpc-request does vis-a-vis handling errors,
timeouts and user quits..

* eglot-tests.el
(eglot--call-with-dirs-and-files): Simplify cleanup logic.
(slow-sync-connection-wait)
(slow-sync-connection-intime, slow-async-connection)
(slow-sync-error): New tests.

* eglot.el (eglot-sync-connect): New defcustom.
(eglot-ensure, eglot): Simplify.
(eglot--connect): Honour eglot-sync-connect.  Complicate
considerably.
(eglot-connect-timeout): New defcustom.
(Package-requires): Require jsonrpc 1.0.6

GitHub-reference: close https://github.com/joaotavora/eglot/issues/68