conn: deprecate Halt()

[maruel]

Move the blocking operation context from the driver to the individual function call.

See https://docs.google.com/document/d/1vQdZdoOMaIan7dKwcAzqHbfM_LCnnSMUQvqypRkjohM for more information.

As such, Halt() will not be needed anymore.

[maruel]

This is a big deal, but the rationale is that it will tremendously simplify the device drivers. Most people get this wrong, and it had been a point of friction for contributors and obscure for users.

[maruel]

Ported to https://github.com/periph/conn/issues/29.