MEDIUM: mux-h2: support emitting CONTINUATION frames after HEADERS

MEDIUM: mux-h2: support emitting CONTINUATION frames after HEADERS

There are some reports of users not being able to pass "enterprise"
traffic through haproxy when using H2 because it doesn't emit CONTINUATION
frames and as such is limited to headers no longer than the negociated
max-frame-size which usually is 16 kB.

This patch implements support form emitting CONTINUATION when a HEADERS
frame cannot fit within a limit of mfs. It does this by first filling a
buffer-wise frame, then truncating it starting from the tail to append
CONTINUATION frames. This makes sure that we can truncate on any byte
without being forced to stop on a header boundary, and ensures that the
common case (no fragmentation) doesn't add any extra cost. By moving
the tail first we make sure that each byte is moved only once, thus the
performance impact remains negligible.

This addresses github issue #249.