Add mailbox client manager

Merged akwizgran requested to merge 2228-mailbox-client-manager into master

This branch adds a mailbox client manager that manages clients for our own mailbox and/or contacts' mailboxes.

The DNS and SOCKS bindings are moved from briar-core to bramble-core so we can make HTTP connections via Tor in bramble-core. I didn't make them part of BrambleCoreModule because we want to use different DNS and SOCKS bindings for FeedManagerIntegrationTest.

Closes #2228 (closed).

Edited by Torsten Grote

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