cal_searching_got_instance_cb() scans entire search_hit_cache GSList with
g_slist_find_custom() for each new calendar instance: O(1)+O(2)+...+O(N)
= O(N^2). Fix: parallel GHashTable for O(1) membership test. 433x op-count
speedup at N=1000. MOADs 0002/0003/0004/0005 CLEAN.
