Merge #16939: p2p: Delay querying DNS seeds

96954d17948662672cababc940e453dff08e8cbb DNS seeds: don't query DNS while network is inactive (Anthony Towns)
fa5894f7f581718ea28bb34b52fcd3b33ff3e644 DNS seeds: wait for 5m instead of 11s if 1000+ peers are known (Anthony Towns)

Pull request description:

  Changes the logic for querying DNS seeds: after this PR, if there's less than 1000 entries in addrman, it will still usually query DNS seeds after 11s (unless the first few peers tried mostly succeed), but if there's more than 1000 entries it won't try DNS seeds until 5 minutes have passed without getting multiple outbound peers. (If there's 0 entries in addrman, it will still immediately query the DNS seeds). Additionally, delays querying DNS seeds while the p2p network is not active.

  Fixes #15434

ACKs for top commit:
  fanquake:
    ACK 96954d17948662672cababc940e453dff08e8cbb - Ran some tests of different scenarios. More documentation is being added in #19084.
  ariard:
    Tested ACK 96954d1, on Debian 9.1. Both MANY_PEERS/FEW_PEERS cases work.
  Sjors:
    tACK 96954d1 (rebased on master) on macOS 10.15.4. It found it useful to run with `-debug=addrman` and change `DNSSEEDS_DELAY_MANY_PEERS` to something lower to test the behaviour, as well as renaming `peers.dat` to test the peer threshold.
  naumenkogs:
    utACK 96954d17948662672cababc940e453dff08e8cbb

Tree-SHA512: 73693db3da73bf8e76c3df9e9c82f0a7fb08049187356eac2575c4ffa455f76548dd1c86a11fc6beea8a3baf0ba020e047bebe927883c731383ec72442356005

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