GeezerVison

Instead of threads and flags, employ high-level concurrency primitives
like tasks and futures.

A thread pool allows to run tests in parallel so it is future-proof.

Start delegating all exceptions to a main, tester thread.

- thread creation for each new client has no use in single-shot
  disposable servers that need to be created fresh for every test case
  anyway

- stop-flag has no use either; there is no interactive session as well
  as waiting for other users. To test server-side connection abruption,
  instruct a server instance in advance

- wrap_conn either modified self.sslconn and returned True, or swallowed
  exceptions and returned False. Now it either return sslconn directly
  or lets an exception to bubble up through a future into a main thread.
  The main thread, in its turn, either expects it with assertRaises() or
  lets it bubble further so the test case is reported as failed

- transparent bubbling up means no explicit collection of error
  messages in conn_errors. Let unittest package do this job for us

- universal read-write-close operations are changed from inner ifs to
  assignment of specific lambda functions

- constant repetition of `if support.verbose and chatty:` //
  `sys.stdout.write(' server: ...\n')` is moved into log()

- chatty and connectionchatty flags are merged to form a single story of
  communication between a client and a server

- the first usage of ThreadedEchoServer is on line 1286 while the server
  itself was declared starting with line 2557; the new implementation
  is put before the first usage for sequential narration

In ThreadedEchoServer chatty=True by default so omit it.

Force clients to wait on a server socket while the server worker thread
starts up, instead of erroring on unavailable host. For this, open
the socket in a main thread.

Server socket timeout has no use because the only operation it limits
is listen(). To have it permawaiting we need a main thread interrupted
without terminating the whole program; that's possible under a debugger
only.

…t breaks it with a long-live thread pool

This reverts commit d10cc72.

… between threads

This reverts commit b410384.