Race condition in sync implemention delays closing connections

[aaugustin]

When this line:

websockets/src/websockets/sync/connection.py

Line 814 in 8c9f6fc

data = self.socket.recv(self.recv_bufsize)

runs before this line:

websockets/src/websockets/sync/client.py

Line 384 in 8c9f6fc

connection.close_socket()

closing the socket doesn't interrupt recv() because recv() isn't running in the main thread — and interruption is based on signals which are only received by the main thread.

First discovered in #1592.