Get rid of extra memory allocations related to `NetMessage:s` in MP games

[ManManson]

There's a constant stream of memory allocations coming from networking layer in MP games. These allocations can be traced to NetMessage instances, each of which has a vector<uint8_t> to store the message payload.

This behavior can be easily observed by profiling heap allocations with any applicable tool (for example: perf, HeapTrack, or MSVC's builtin profiling tools) while running a WZ process.

It probably makes sense to provide a custom allocation strategy for the message payload data (e.g. a memory pool-based allocator/arena allocation or dynamically growing circular buffer).

Also, there's a chance that some NetMessage copies can be avoided (but I'm not yet sure about that), so an additional investigation should be conducted to either prove or disprove that.