Rewrite all time-based checks to use microtime

Problem/Motivation

The module currently relies upon the time() function to track when various caches are cleared. The time() function tracks events to the nearest second. The problem is that there can be thousands of operations within an individual second, so a second becomes an extremely imprecise value to work from. The current system leads to issues like #2444221: cache_lifetime should not affect cache_clear_all() → , whereby it's impossible to discern between cache_set() being called before or after a cache_clear_all().

Proposed resolution

Change all time-based logic to use microtime() instead of time().

Remaining tasks

Write a patch, test it.

User interface changes

None.

API changes

None.

Data model changes

Additional variables are kept to track the microtime of various events in addition to the timestamp.