activeExpireCycle(): better precision in max time used.
activeExpireCycle(): better precision in max time used.

activeExpireCycle() can consume no more than a few milliseconds per
iteration. This commit improves the precision of the check for the time
elapsed in two ways:

1) We check every 16 iterations instead of the main loop instead of 256.
2) We reset iterations at the start of the function and not every time
   we switch to the next database, so the check is correctly performed
   every 16 iterations.

JackieXie168/redis,JackieXie168/redis,JackieXie168/redis,JackieXie168/redis