Kill server's output and events buffers from eglot-shutdown ()

Kill server's output and events buffers from eglot-shutdown ()

* eglot.el
(Package-Requires): Require jsonrpc 1.0.5
(eglot-shutdown): Kill events and stderr buffers of the
server, unless new PRESERVE-BUFFERS argument is non-nil.
eglot-reconnect): Preserve buffers on shutdown.

* eglot-tests.el (eglot--call-with-dirs-and-files): Call
eglot-shutdown with non-nil PRESERVE-BUFFERS arg.

GitHub-reference: https://github.com/joaotavora/eglot/issues/66