Make TCP accept AsyncRead + AsyncWrite

[dragonnn]

Hi!
Would you accept a PR with reworks Modbus TCP to allow to use AsyncRead + AsyncWrite?
As a TcpStream has too AsyncRead + AsyncWrite so that shouldn't be hard to do.
Of course we could keep the current connect and connect_slave and just add connect_slave_from_transport or something like that.
My use case for that:

• doing some connecting setup before starting working with tokio-modbus, I have connections where I first need to send a 'login' packet to have access to modbus
• running modbus-tcp over udp, not a typical use case but I sometimes have to do that

[uklotzde]

This is a prerequisite for #133.