Performance improvement: make "apm_version_names_compare" faster by a factor of ~100
The function "apm_version_names_compare" is called relatively often and issues an SQL query. The function is stable (returns always the same results for the same input) and called with only a few different input values. By using acs::per_thread_cache the performance improves from 265 microseconds to 2 microseconds; by using the per-tread cache the operation scales better since it is fully lock-free (once cached).