Prior to this change, the unit tests' echo servers' connections would sometimes
linger beyond server termination. A timeout mechanism is now implemented, which
will terminate a connection and clean up its resources when the timeout is
reached. For the purpose of unit testing, test echo servers now assert that
all connections have been terminated when the servers are closed.
For threaded echo servers, the timeout mechanism involves the use of sockets
with timeouts instead of blocking sockets. This required an implementation with
the proper handling of timeout sockets at the sslconnection level.
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