BUG-8494: Cap queue sleep time

BUG-8494: Cap queue sleep time

Inconsistency in transmit queue backpressure has caused an observed
delay of over 44 minutes in CSIT. Thile this is an artefact of a bug,
we should not delay application thread for large periods of time, as
that can cause failures in the application (if it has other
time-bound duties like BGP keepalive messages and similar).

Restrict the sleep time to 5 seconds, emitting an INFO-level messages
when we reach this level of sleeping. This should not typically not
occur, as the backpressure should be kicking in much sooner, smooting
out the delay curve.

Change-Id: Ie5f148248caa71791bdda71ddd7e33e5733aa7f8
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>