Use a time-based limit for the Rock Store rebuild loop.

Use a time-based limit for the Rock Store rebuild loop.

Compared to the old counter-based limit, a time-based limit is better because
it allows us to quit right after a blocked I/O or quit sooner when I/Os become
slower. Similarly, a time-based limit often allows us to rebuild a much larger
number of entries at a time because most sequential read I/Os are very fast.