[core] h2: avoid sending tiny DATA frames
[core] h2: avoid sending tiny DATA frames

h2: avoid sending tiny DATA frames when h2 window is tiny
and a larger amount of data is pending to be sent;
wait for slightly larger window to be available

note: must temporarily disable this when running h2spec since some
      h2spec tests expect 1-byte DATA frame, not a deferred response

gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4