New "deferred requests" that wait until server is ready

New "deferred requests" that wait until server is ready

Calling textDocument/hover or textDocument/documentHighlight before
the server has had a chance to process a textDocument/didChange is
normally useless. The matter is worse for servers like RLS which only
become ready much later and send a special notif for it (see
https://github.com/rust-lang-nursery/rls/issues/725).

So, keeping the same coding style add a DEFERRED arg to eglot--request
that makes it maybe not run the function immediately. Add a bunch of
logic for probing readiness of servers.

* README.md: Update

* eglot.el (eglot--deferred-actions): New process-local var.
(eglot--process-filter): Call deferred actions.
(eglot--request): Rewrite.
(eglot--sync-request): Rewrite.
(eglot--call-deferred, eglot--ready-predicates)
(eglot--server-ready-p): New helpers.
(eglot--signal-textDocument/didChange): Set spinner and call
deferred actions.
(eglot-completion-at-point): Pass DEFERRED to eglot-sync-request.
(eglot-eldoc-function): Pass DEFERRED to eglot-request
(eglot--rls-probably-ready-for-p): New helper.
(rust-mode-hook): Add eglot--setup-rls-idiosyncrasies
(eglot--setup-rls-idiosyncrasies): New helper.