MINOR: mux-h2: add tune.h2.be.rxbuf and tune.h2.fe.rxbuf global settings
These settings allow to change the total buffer size allocated to the
backend and frontend respectively. This way it's no longer necessary to
play with tune.bufsize nor increase the number of streams to benefit from
more buffers.
Setting tune.h2.fe.rxbuf to 4m to match a sender's max tcp_wmem resulted
in 257 Mbps for a single stream at 103ms vs 121 Mbps default (or 5.1 Mbps
with a single buffer and 64kB window).