MINOR: ncbmbuf: implement advance operation

MINOR: ncbmbuf: implement advance operation

Implement ncbmb_advance() function for the ncbmbuf type. This allows to
remove bytes in front of the buffer, regardless of the existing gaps.
This is implemented by resetting the corresponding bits of the bitmap.

As the previous patch, this commit must be backported prior to the fix
to come on QUIC CRYPTO frames parsing.