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).