vinyl: warn when transaction waits for quota for too long

If a vinyl transaction stalls waiting for quota for more than
box.cfg.too_long_threshold seconds, emit a warning to the log:

  W> waited for 699089 bytes of vinyl memory quota for too long: 0.504 sec

This will help us understand whether our users experience lags
due to absence of throttling in vinyl (see #1862).

Closes #3096