[core] perf: adjust max h2 stream send increment
[core] perf: adjust max h2 stream send increment

subtract 9 byte HTTP/2 frame overhead from each 16k DATA
frame for more efficient sending of large files

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