Upload performance is poor when BDP is higher than the flow-control window.
Previously, the server's receive window was fixed at 64KB, which resulted in
very poor performance for high-BDP links. The receive window now defaults to
1MB and is configurable. The per-connection and per-stream windows are
configurable separately (both default to 1MB as suggested in golang/go#16512).

Previously, the server created a "fixedBuffer" for each request body. This is no
longer a good idea because a fixedBuffer has fixed size, which means individual
streams cannot use varying amounts of the available connection window. To
overcome this limitation, I replaced fixedBuffer with "dataBuffer", which grows
and shrinks based on current usage. The worst-case fragmentation of dataBuffer
is 32KB wasted memory per stream, but I expect that worst-case will be rare.

A slightly modified version of adg@'s grpcbench program shows a dramatic
improvement when increasing from a 64KB window to a 1MB window, especially at
higher latencies (i.e., higher BDPs). Network latency was simulated with netem,
e.g., `tc qdisc add dev lo root netem delay 16ms`.

Duration        Latency Proto           H2 Window

11ms±4.05ms     0s      HTTP/1.1        -
17ms±1.95ms     0s      HTTP/2.0        65535
8ms±1.75ms      0s      HTTP/2.0        1048576

10ms±1.49ms     1ms     HTTP/1.1        -
47ms±2.91ms     1ms     HTTP/2.0        65535
10ms±1.77ms     1ms     HTTP/2.0        1048576

15ms±1.69ms     2ms     HTTP/1.1        -
88ms±11.29ms    2ms     HTTP/2.0        65535
15ms±1.18ms     2ms     HTTP/2.0        1048576

23ms±1.42ms     4ms     HTTP/1.1        -
152ms±0.77ms    4ms     HTTP/2.0        65535
23ms±0.94ms     4ms     HTTP/2.0        1048576

40ms±1.54ms     8ms     HTTP/1.1        -
288ms±1.67ms    8ms     HTTP/2.0        65535
39ms±1.29ms     8ms     HTTP/2.0        1048576

72ms±1.13ms     16ms    HTTP/1.1        -
559ms±0.68ms    16ms    HTTP/2.0        65535
71ms±1.12ms     16ms    HTTP/2.0        1048576

136ms±1.15ms    32ms    HTTP/1.1        -
1104ms±1.62ms   32ms    HTTP/2.0        65535
135ms±0.96ms    32ms    HTTP/2.0        1048576

264ms±0.95ms    64ms    HTTP/1.1        -
2191ms±2.08ms   64ms    HTTP/2.0        65535
263ms±1.57ms    64ms    HTTP/2.0        1048576

Fixes golang/go#16512
Updates golang/go#17985
Updates golang/go#18404

Change-Id: Ied385aa94588337e98dad9475cf2ece2f39ba346
Reviewed-on: https://go-review.googlesource.com/37226Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>