[tailscale] net/http/internal/http2: let request-write goroutines exit early

[bradfitz]

The goroutine spawned per RoundTrip to write the request previously
parked until the stream ended, even after the request was fully sent,
just to wait for the stream-end events and run cleanupWriteRequest.
For clients with many concurrent long-lived response streams (long
polls, event streams), that's a parked goroutine and its stack per
stream doing nothing, which adds up to a large fraction of such a
client's memory use.

Once the request is fully sent, detach: the goroutine exits, and
cleanupWriteRequest instead runs (on a short-lived goroutine) from
whichever stream-end event fires first. Both stream-end events (peer
half-close and abort) are raised under cc.mu, so a mutex-guarded flag
gives exactly-once cleanup dispatch. Request context cancellation is
watched via context.AfterFunc, and ResponseHeaderTimeout is enforced
by a time.AfterFunc timer armed at detach and disarmed when response
headers arrive. Only the deprecated Request.Cancel channel still
requires a parked goroutine to watch it, so requests using it keep
the previous behavior.

Intended for upstreaming; carried in the Tailscale fork until then.

Updates tailscale/corp#29053