net/http/httputil: rewrite flushing code, disable on Server-Sent Events
* Rewrite the flushing code to not use a persistent goroutine, which
also simplifies testing.
* Define the meaning of a negative flush interval. Its meaning doesn't
change, but now it's locked in, and then we can use it to optimize
the performance of the non-buffered case to avoid use of an AfterFunc.
* Support (internal-only) special casing of FlushInterval values per
request/response.
* For now, treat Server-Sent Event responses as unbuffered. (or rather,
immediately flushed from the buffer per-write)