feat: Improve statistics for remote hits/misses

feat: Improve statistics for remote hits/misses

ccache collects statistics about local and remote storage layer get/put
operations and describes them as "local/remote hits/misses" in the
output of "ccache -s". However, since "hits" and "misses" mean "result
hit/miss" in the "cacheable calls" section, it's easy to think that they
measure the same thing.

This commit improves the situation by:

- Adding new "local/remote hits/misses" counters that mean "result
  hit/miss". These are shown if remote storage is used (since they
  otherwise are redundant and equal to the normal "hits/misses"
  counters) or if the -v/--verbose option is given.
- Presenting the previous "local/remote hits/misses" counters as
  "local/remote reads". Only shown in verbose mode.
- Adding "local/remote writes" counters. Only shown in verbose mode.

Closes #1016.