Significantly trimmed down version of an ID<=>IP cache.
Besides acknowledging timeouts, the module isn't trying to do anything fancy with the data besides storing and retrieving.
Assoc's array is now allocated dynamically and per default much smaller (320 entries).
id_hash() was not at all working as expected for very small bucket size (when (size / 4) was zero). Simplified to be trivially correct.
Also added a used flag on adding an entry, which is set by callers if they have that association in active use. Those get priority over unused entries on collision.
Fleshed out test to be at least elementary useful.
Each group chat now uses an own, small assoc (80 entries).
Merge pull request #650 from FullName/ID-IP-basic

Significantly trimmed down version of an ID<=>IP cache.