Cache PacketType#hashCode

[astei]

This should result in a general performance improvement, as PacketType is often used as a map key across ProtocolLib. This primarily will make a difference with large numbers of players or lots of packets being sent by the server.

[dmulloy2]

Not that it makes a whole lot of difference, but any reason for replacing the hashCode implementation entirely instead of just caching the original?

[astei]

The replacement implementation is almost equivalent to the original Objects.hash() version, which forwards to Arrays.hashCode(). I used the "recipe" from Effective Java to write this particular hash code function.