Add peek/peek_finalize to VariablePacket

[dpmkl]

VariablePacket::peek was refactored from parse to read the protocol header and additionally return the header bytes ( ignored in parse but used in peek_finalize ).
VariablePacket::peek_finialize reads the rest of the packet returning a decoded packet and the entire buffer.
VariablePacket::parse uses peek to read the header, no braking changes for anything existing.

The use case for this is that as a broker/bridge you want to be able to early reject connections by peeking into the fixed header if the control type is relevant to the current situation, logging/aborting/exiting without processing the remaining data. To avoid unnecessary encoding/decoding or rereading the buffer in order to forward the packet, the entire packet buffer is returned in peek_finalize.

Added basic tests for VariablePacket::parse and VariablePacket::peek(_finalize)

This affects only tokio async use.

[zonyitoo]

LGTM. Will be released in v0.6.0