........S. [ZBX-8414] fixed multiple issues caught while writing value cache unit tests:
* incorreect statistics update when database contains only part of data requested by count based request
* single value request returns FAIL in low memory mode when database contains not requested values. It
  should return SUCCEED while setting 'found' flag to zero
* with extremaly rage timeshifts (resulting timestamp value is less than seconds per month) value cache
  would try negative clock queries by count based requests
* it was theoretically possible for single value request to fail caching value if the requested timestamp
  matched the last cached value timestamp.