Clean up and do fixes to hash functions and newly introduced murmur3 hashes in #61934
* Clean up usage of murmur3
* Fixed usages of binary murmur3 on floats (this is invalid)
* Changed DJB2 to use xor (which seems to be better)
Move multiplayer classes to "core/multiplayer" subdir.
Move the RPCConfig and enums (TransferMode, RPCMode) to a separate
file (multiplayer.h), and bind them to the global namespace.
Move the RPC handling code to its own class (RPCManager).
Renames "get_rpc_sender_id" to "get_remote_sender_id".
