Placing more than one airdrop on the same block results in only one chest showing up.

[dpvb]

Dont know how to handle this yet but this should be fixed.

[ms5984]

We can use a set of vectors--all we have to do is test presence in AirdropManager#startAirdrop; perhaps the return value can be a boolean (false if a matching vector is found in the set and thus an airdrop is already happening on that location). Vectors will be removed from the set when AirdropManager#createAirdropChest is called.