cache garbage collector

Experimental code for cache GC.

I've set the defaults to values that seemed to work well (clear about ~10% of cache when 80% full, use 100 operations per transaction). Higher values for allowed cache usage didn't seem to properly prevent complete cache flush. High values of ops per transaction (or unlimited) caused issues as well.

Closes #257 (closed)

lib/cache/impl.h

@@ -63,6 +63,7 @@ struct entry_apex;
 /** Check basic consistency of entry_h for 'E' entries, not looking into ->data.
  * (for is_packet the length of data is checked)
  */
+KR_EXPORT
 struct entry_h * entry_h_consistent_E(knot_db_val_t data, uint16_t type);
 
 struct entry_apex * entry_apex_consistent(knot_db_val_t val);