src/utils_cache.[ch]: Add uc_get_history[_by_name].

src/utils_cache.[ch]: Add uc_get_history[_by_name].

These two new functions can be used to get historical data of values in
the cache. This can be used to calculate floating averages, hysteresis
and a shipload of other aggregation and consolidation functions.

The current implementation is probably not yet perfect:

 - If not enough values are available to satisfy the request, the buffer
   will be enlarged and NaNs will be returned in the newly allocated
   cells. The caller has no way to recognize this case.

 - If a value is missing, no NaNs will be added to the cache. It's
   unclear if this was desirable.

 - The returned values are reversed, i. e. val[0] will be the newest
   value, val[n-1] will be the oldest. Here, too, I'm unsure which way
   is easier to comprehend / use. I went for this implementation because
   it was easier to write.