QUIC: refactored CRYPTO and STREAM buffer ordering.

QUIC: refactored CRYPTO and STREAM buffer ordering.

Generic function ngx_quic_order_bufs() is introduced.  This function creates
and maintains a chain of buffers with holes.  Holes are marked with b->sync
flag.  Several buffers and holes in this chain may share the same underlying
memory buffer.

When processing STREAM frames with this function, frame data is copied only
once to the right place in the stream input chain.  Previously data could
be copied twice.  First when buffering an out-of-order frame data, and then
when filling stream buffer from ordered frame queue.  Now there's only one
data chain for both tasks.