added SSL request and SASL auth messages

[BOPOHA]

func ReadNBytes

I am facing the problem of reading a stream into a byte chunk of the full size of the pg message.
But it works fine with byte-by-byte reading.
You can reproduce this error by running a multi-line query with 128+ KB response.
Also I found that the problem occurs where the TCP length changes (from 8k to 32k bytes in my case).

func ParseClientMessage / ParseServerMessage

small refactoring, and we pass the prepared message to the parsing functions.

errorMap:

changed the struct for correct parsing by the messageToString function

Added:

• SSLRequest
• SASL messages