Continuous-mode use/attack don't give correct rewards in multi-agent missions.

[timhutton]

The second half of two_diggers.py (currently on #302) is failing because the agents get rewards of 2 and 0, instead of 11 and 110.

Problem 1: Rewards for collecting item are all being sent to the first client, in continuous mode.
Problem 2: Rewards for discarding item are not being triggered at all when placing an item in continuous mode. (As with the discrete case, to avoid reward loopholes where an agent repeatedly gets the reward for collecting by placing and digging, we need use to trigger the discarding reward.)

[timhutton]

Looks like EntityItemPickupEvent is server-side only. link

Suggestion is to use ItemPickupEvent instead. Update: no, is server-side too.