To make eclipse attacks more difficult, regularly initiate outbound connections and stay connected long enough to sync headers and potentially learn of new blocks. If we learn a new block, rotate out an existing block-relay peer in favor of the new peer. This augments the existing outbound peer rotation that exists -- currently we make new full-relay connections when our tip is stale, which we disconnect after waiting a small time to see if we learn a new block. As block-relay connections use minimal bandwidth, we can make these connections regularly and not just when our tip is stale. Like feeler connections, these connections are not aggressive; whenever our timer fires (once every 5 minutes on average), we'll try to initiate a new block-relay connection as described, but if we fail to connect we just wait for our timer to fire again before repeating with a new peer.
Backport of core#19858.
Ref T1696.