WARNING should be WARN.

Normally stream is in dependency tree, but sometimes it isn't.  For
example, client does not put pushed reserved stream in dependency
tree.

This is not obvious but it makes intermediaries flush and forward DATA
frame boundary without excessive buffering.  Since we have different
TCP connections frontend and backend, this may not work.  This is
still experimental.

Use the same behaviour the current Google server does: start with 1300
TLS record size and after transmitting 1MiB, change record size to
16384.  After 1 second idle time, reset to 1300.  Only applies to
HTTP/2 and SPDY upstream connections.