refactor: simplify how the Login7Payload is converted to a buffer
refactor: simplify how the Login7Payload is converted to a buffer

We now allocate a single 94 byte buffer to hold all the fixed data, and 
later concat this buffer with the data buffers together. This allows us 
to completely move away from using `WritableTrackingBuffer`s for this 
operation, while also making the code a bit more straightforward.

tediousjs/tedious,tediousjs/tedious,pekim/tedious