net/http/fcgi: eliminate race, keep request id until end of stdin

There was a race condition that could lead to child.serveRequest
removing the request ID before child.handleRequest had read the empty
FCGI_STDIN message that indicates end-of-stream which in turn could
lead to child.serveRequest blocking while trying to consume the
request body.

Now, we remove the request ID from within child.handleRequest after
the end of stdin has been detected, eliminating the race condition.

Since there are no more concurrent modifications/accesses
to child.requests, we remove the accompanying sync.Mutex.

Change-Id: I80c68e65904a988dfa9e3cceec1829496628ff34
GitHub-Last-Rev: b3976111ae1d3bbbfa36045f99acce7911a18c44
GitHub-Pull-Request: golang/go#42840
Reviewed-on: https://go-review.googlesource.com/c/go/+/273366


Trust: Damien Neil <dneil@google.com>
Trust: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>