Limit of concurrent splits should be a lot smaller than 256

[TwistedAsylumMC]

https://github.com/Sandertv/go-raknet/blob/fbd2ad1000cdbe1411158d6f8a5dae1acb2bd978/conn.go#L666-L669

Theoretically speaking the client should never send more than one or two unique split IDs, and go-raknet allows up to 256 at one time. This could be abused to keep ~97mb (256*255*1492) of data in memory from one connection if they don't send the last split for each packet.