Merge #17164: p2p: Avoid allocating memory for addrKnown where we don't need it

b6d2183858 Minor refactoring to remove implied m_addr_relay_peer. (User)
a552e8477c added asserts to check m_addr_known when it's used (User)
090b75c14b p2p: Avoid allocating memory for addrKnown where we don't need it (User)

Pull request description:

  We should allocate memory for addrKnown filter only for those peers which are expected to participate in address relay.

  Currently, we do it for all peers (including SPV and block-relay-only),  which results in extra RAM where it's not needed.

  Upd:
  In future, we would still allow SPVs to ask for addrs, so allocation still will be done by default.
  However, they will be able to opt-out via [this proposal](https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2019-October/017428.html) and then we could save some more memory.
  This PR still saves memory for block-relay-only peers immediately after merging.

Top commit has no ACKs.

Tree-SHA512: e84d93b2615556d466f5ca0e543580fde763911a3bfea3127c493ddfaba8f05c8605cb94ff795d165af542b594400995a2c51338185c298581408687e7812463

src/net.cpp

@@ -2666,11 +2666,10 @@ CNode::CNode(NodeId idIn, ServiceFlags nLocalServicesIn, int nMyStartingHeightIn
     addrBind(addrBindIn),
     fInbound(fInboundIn),
     nKeyedNetGroup(nKeyedNetGroupIn),
-    addrKnown(5000, 0.001),
     // Don't relay addr messages to peers that we connect to as block-relay-only
     // peers (to prevent adversaries from inferring these links from addr
     // traffic).
-    m_addr_relay_peer(!block_relay_only),
+    m_addr_known{block_relay_only ? nullptr : MakeUnique<CRollingBloomFilter>(5000, 0.001)},
     id(idIn),
     nLocalHostNonce(nLocalHostNonceIn),
     nLocalServices(nLocalServicesIn),