Drop the ServerHandler class

[natebosch]

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Based on the implementation it seems to exist to allow a late binding between a Server and a Handler. The comment mentions "This is used to expose a virtual [Server] that's actually one part of a larger URL-space." but there is no code related to partitioning URLs.

I cannot find any uses, and I can't think of a particular reason a late binding would be useful. If it does turn out to be useful I think we could write it in a more straightforward way.

We should deprecate the class for now, and remove it in the next major version.

[kevmoo]

Agreed. I asked about this a while ago and came to a similar conclusion!