The existing Framer in net/http2 allocates a new DataFrame struct
for each DataFrame read on calls to ReadFrame. The SetReuseFrame
option introduced here, if set on a Framer, allows the
Framer to reuse Frame objects and changes the ReadFrame API
so that returned Frame objects are only valid until the next call
to ReadFrame. This opt-in API now only implements reuse of DataFrames,
but it allows the Framer to reuse of any type of Frame.

The footprint caused by creation of new DataFrame structs per data
frame was noticed in micro benchmarks of "gRPC" server "streaming
throuhgput", which uses the Framer in this package. This benchmark
happened to use long lived http2 streams that do client-server "ping-pong"
requests with small data frames, and DataFrames were seen to be a
significant source of allocations.

Running local benchmarks with: (from x/net/http2 directory)

$ go test -run=^$ -bench=BenchmarkServerToClientStream

example output:
* expect an alloc reduction of at least 1 and a small memory reduction between
"BenchmarkServerToClientStreamDefaultOptions" and
"BenchmarkServerToClientStreamReuseFrames"

BenchmarkServerToClientStreamDefaultOptions-12    	   30000
46216 ns/op	     971 B/op	      17 allocs/op
BenchmarkServerToClientStreamReuseFrames-12       	   30000
44952 ns/op	     924 B/op	      16 allocs/op

Fixes golang/go#18502

Change-Id: Iad93420ef6c3918f54249d867098f1dadfa324d8
Reviewed-on: https://go-review.googlesource.com/34812
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>