heartbeat_check in kombu.connection is not reraising exceptions as
exceptions.OperationalError, and the socket timeout during the heartbeat
check is really an issue seen in the field when a node goes down; the
heartbeat thread just tries again and again to deal with it, without
success.
Change-Id: I26dbdb18a7e64946db2cba676764ff2d428c7897
Closes-Bug: #1657444