worker: honor the restart boolean before checking heartbeat
worker: honor the restart boolean before checking heartbeat

Change-Id: Ib260b870d918afd5f2b5ab0566c3064a01c3f161
(cherry picked from commit ff5e00073a72b3df54027b4f30e808cd6e2239f2)
Linaro/lava-server,Linaro/lava-server,Linaro/lava-server,Linaro/lava-server