HTTP Proxy: Separate Saving Jump Service Addresses from Message Processing

[coneiric]

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After IRC collaboration with @anonimal & @brbzull0, it seems to me that HTTPMessage::SaveJumpServiceAddress should really be its own async proxy handler, HTTPProxyHandler::SaveAddress.

Refactoring saving addresses as a proxy handler will decouple HTTP message processing from AddressBook's timer + mutex (allowing us to get rid of the hack needed in #838).

References #340 & #844.

[coneiric]

Would it be better design to have the handler send the address over the socket as a NetDB DatabaseStoreMessage, or directly insert to the AddressBook?

edit: NetDB is a different submodule, should insert to AddressBook from an async proxy handler, after verifying the address is reachable. Thanks for clarification over IRC @anonimal 👍

Thought you might have been hinting at the former on IRC @anonimal (was mistaken, see above):

@anonimal Right off the bat, IMHO that should be at least 4 different classes. The socket/handler could hypothetically be templated, not specific to HTTP but we'll ignore that for now.
@anonimal From there you can almost certainly decouple the work even further.
@anonimal In that scheme, I don't see a reliance on inheritance but really there are multiple ways to deal with the issue.
@anonimal Did that make sense?
@anonimal Notice how I left out addressbook and storage, etc. This is intentional.

[anonimal]

NOTICE: THIS ISSUE HAS BEEN MOVED TO GitLab. Please continue the discussion there. See #1013 for details.