MNDR:
- rename MYSQLND_NET to MYSQLND_PPEC (protocol packet envelope codec).
PPEC does only the encoding and decoding to the protocol frame.

MNDR:
- intermediate step to move MYSQLND_NET from mysqlnd_vio.c to mysqlnd_wireprotocol.c
In following commits MYSQLND_NET's methods and data will be moved to MYSQLND_PROTOCOL

MNDR:
- moved networking code to mysqlnd_vio.c