MINOR: quic: simplify length calculation for STREAM/CRYPTO frames

MINOR: quic: simplify length calculation for STREAM/CRYPTO frames

STREAM and CRYPTO frames have a similar encoding format. In particular,
both of them have a variable-length integer Length field just before the
frame payload.

It is complex to determine the optimal Length value before copying the
payload data in the remaining buffer space. As such, helper functions
were implemented to calculate this. However, CRYPTO and STREAM frames
encoding implementation were not completely aligned, which renders the
code harder to follow.

The purpose of this commit is to simplify CRYPTO and STREAM frames
encoding. First, a new helper quic_int_cap_length() is defined which is
useful to determine the optimal buffer room available if prefixed by a
variable-length integer as Length field. Then, processing of both CRYPTO
and STREAM frames is now nearly identical, based on this new helper
function. Functions max_available_room() and max_stream_data_size() are
now unused and are removed.