x/net/http2: use Context

[bradfitz]

Use Context in http2

cmd/bundle's lack of +build tag support might make it trickier.

(Parent bug: #14660)

/cc @adg

[gopherbot]

CL https://golang.org/cl/23002 mentions this issue.

[gopherbot]

CL https://golang.org/cl/23003 mentions this issue.

[gopherbot]

CL https://golang.org/cl/23221 mentions this issue.

[gopherbot]

CL https://golang.org/cl/23220 mentions this issue.