the tcp_server's EXPECTED() packet size to be uint32_t, to be consistent with blob.h

* make EXPECTED() to cast the first byte of the buffer to uint32_t
* make tcp_client->pos to be uint32_t

this way we are consistent that what we expect from the stream is exactly
the same kind of thing that we use in blob.h (size sign wise)

also simplifies the code a bit, removing unnecessary checks