Add caches with short lived entries for disco#info and disco#items results

Sometimes the same entity and node pair is queried multiple times in a brief period. Assuming that entities typically do not change their disco#info and disco#items related capabilities, we could avoid a network I/O if Smack would cache the results for a short period (30 seconds, 1 minute, 5 minutes),

Note that until DiscoverInfo and DiscoverItems is mutable, the need to return cloned instances of the classes.

Methods where the caches should be plugged in

• ServiceDiscoveryManager.discoverInfo(Jid, String)

• ServiceDiscoveryManager.discoverItems(Jid, String)

Introducing caching may cause inconsistency issues. I suspect those are especially problematic for disco#items. Hence this feature should be introduced with a knob to enable and disable it. Potentially even a matrix setting where one dimension is disco#info and disco#items, and the other one is without-node and with-node.

Links:

https://github.com/Flowdalic/Smack/pull/21#issuecomment-471281503

Branch:

https://github.com/Flowdalic/Smack/tree/disco-info-items-cache