worker: let a stop request interrupt the reconnect backoff

The retry sleep polled only fShutdown, while the loop around it polls
fShutdown, fGenerateBitcoins and nMineMode. That was survivable at a flat
10s. Raising the ceiling to 120s made it expensive: after an operator
cleared Generate or switched to Relay mode, the participant session --
and the vfThreadRunning[3] slot that gates restarting it -- stayed held
for up to two minutes.

Measured with the loop shape replicated and the stop condition flipped 1s
into a 120s sleep: 119.01s to notice before, 1.00s after, via either
fGenerateBitcoins or nMineMode. The fShutdown path is unchanged at 1.00s.

It also narrows the window on a pre-existing unsynchronised read of
vfThreadRunning[3] at gui.cpp:901, where a stop-then-start inside it
leaves mining silently not restarted.

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