Re-Implement X-ApiKey so that it doesn't rely on transport encryption

#534 Implemented the X-ApiKey feature which I didn't pay attention to until it was already implemented

In principle it's a useful feature to have, but its current implementation is insecure as it leaks the credentials and is therefore even weaker than our default http-digest-auth based scheme (which isn't state of the art either, being MD5 based and with some minor other flaws, but it's better than nothing).

However, `X-ApiKey` can be fixed by using the same technique that e.g. AWS uses for its Authorization headers (a MAC-based scheme); Even the older AWS v2 scheme would be an improvement, although I'd rather go for the AWS v4 scheme.

I'm marking this high priority besides for the obvious security issue, because once implemented, every client using the old insecure X-ApiKey scheme will be broken, hence it's critical to fix this before we have too many users.

I'm prepared to implement this myself, because I care too much about this being done properly.