vinyl: unthrottle scheduler on manual checkpoint

Before this patch box.snapshot() bails out immediately if it sees
that the scheduler is throttled due to errors. For instance:

box.error.injection.set('ERRINJ_VY_RUN_WRITE', true)
snapshot() -- fails due to ERRINJ_VY_RUN_WRITE
box.error.injection.set('ERRINJ_VY_RUN_WRITE', false)
snapshot() -- still fails despite the fact that injected error is unset

As a result, one has to wait up to a minute to make a snapshot. The
reason why throttling was introduced was to avoid flooding the log
in case of repeating disk errors.
What is more, to deal with schedule throttling in tests, we had to
introduce a new error injection (ERRINJ_VY_SCHED_TIMEOUT). It reduces
time duration during which the scheduler remains throttled, which is
ugly and race prone.  So, let's unthrottle scheduler when checkpoint
process is launched via manual box.snapshot() invocation.

Closes #3519