peer: modify readMessage/writeMessage to be message oriented
peer: modify readMessage/writeMessage to be message oriented

This commit modifies both readMessage and writeMessage to be further
message oriented. This means that message will be read and written _as
a whole_ rather than piece wise. This also fixes two bugs: the
readHandler could be blocked due to an sync read, and the writeHandler
would unnecessarily chunk up wire messages into distinct crypto
messages rather than writing it in one swoop.

Also with these series of changes, we’re now able to properly parse
messages that have been padded out with additional data as is allowed
by the current specification draft.

ddustin/lnd,aakselrod/lnd,ddustin/lnd,AndrewSamokhvalov/lnd,LightningNetwork/lnd,LightningNetwork/lnd,AndrewSamokhvalov/lnd,aakselrod/lnd