Expose actual number of bytes transferred in ReadRawData and possibly other commands

[pepijndevos]

On my Rigol scope, when using the LXI transport, the scope sometimes sends less data than requested, which leads to stale data from another channel in a temp buffer being rendered, see https://github.com/azonenberg/scopehal-apps/issues/146#issuecomment-663620558

To correctly fix this issue, these commands need to expose the actual number of bytes read (and written?)

I suppose as of yet, no other scope is so broken that it's a big problem, but I think that for API stability and correctness sake, any network IO function should be prepared to handle incomplete data and report the number of bytes transferred.

[azonenberg]

All of the other transports use Socket::ReadLooped/WriteLooped(), which call read(2) and write(2) in a loop until exactly the requested number of bytes have been sent.

So while they can hang, they can never return partial data. Except in the case of a socket timeout, which we probably should handle.