Add a new config option that allows to throttle the rate of upload
requests. The new default behavior is to initiate a new upload request
every 10 milliseconds max, before there was no limit.

The old behavior can be restored by setting a lower value, or setting
`-1`. Due to quirks in config parsing, `0` means the same as `10`.

On my machine, `10` ms improves stability on splithttp+h2. It does not
affect QUIC at all. More testing is needed.

Since some CDN also bill by number of requests (on top of bytes
transferred), the combination of `maxConcurrentUploads` and
`maxUploadInterval` should also provide some more control over the CDN
cost. (In theory, but I don't have that usecase)