MINOR: quic: refactor STREAM encoding and splitting

MINOR: quic: refactor STREAM encoding and splitting

CRYPTO and STREAM frames encoding is similar. If payload is too large,
frame will be splitted and only the first payload part will be written
in the output QUIC packet. This process is complexified by the presence
of a variable-length integer Length field prior to the payload.

This commit aims at refactor these operations. Define two functions to
simplify the code :
* quic_strm_frm_fillbuf() which is used to calculate the optimal frame
  length of a STREAM/CRYPTO frame with its payload in a buffer
* quic_strm_frm_split() which is used to split the frame payload if
  buffer is too small

With this patch, both functions are now implemented for STREAM encoding.