Fix a Tramp bug when running several asynchronous processes

Fix a Tramp bug when running several asynchronous processes

* lisp/net/tramp-cache.el (tramp-get-hash-table):
Add ;;;###tramp-autoload cookie.

* lisp/net/tramp.el (tramp-file-name-handler): Flush connection
properties "process-name" and "process-buffer".

* test/lisp/net/tramp-tests.el
(tramp--test-deftest-direct-async-process): Skip when underlying
TEST has taken too much time.
(tramp--test-with-proper-process-name-and-buffer): Remove.
(tramp-test45-asynchronous-requests): Remove callees.

(cherry picked from commit 0b7f649614d8eb7694e98372dea7b7e01f090265)