vinyl: fix memory dump trigger

vy_quota_signal() doesn't wake up a consumer if it won't be able to
proceed because of the memory limit. This is OK, but it doesn't attempt
to trigger memory dump in this case either. As a result, it may occur
that dump isn't triggered and all waiting consumers are aborted by
timeout.  E.g. this happens if memory dump releases no memory, which is
possible because memory is allocated and freed in 16 MB chunks. This
results in occasional vinyl/quota_tmeout test failures.

Fix this by moving the dump trigger right in vy_quota_may_use() so that
it's called whenever we consider a consumer for wakeup.