Handle managed buffers in own process var

Handle managed buffers in own process var

This should save some trouble when testing noninteractively.  Because
eglot--shutdown didn't turn off the minor mode, test code running
immediately after it could still make didClose requests, for example.
The sentinel was the previous responsible for turning off the minor
mode and didn't get a chance to run in that case. Now eglot--shutdown
is also responsible for turning off the minor mode.

All this should be hidden behind eglot--managed-mode-onoff.

* eglot.el (eglot--managed-buffers): New process-local variable.
(eglot--process-sentinel): Turn off managed mode.
(eglot--managed-mode-onoff): New function.
(eglot--managed-mode): Don't offer to kill server here.
(eglot--buffer-managed-p): Remove.
(eglot--maybe-activate-editing-mode): Activate mode here.
(eglot-shutdown): Turn off minor mode here.
(eglot--server-window/progress): Simplify slightly.