Replace relevant services logic with a function suite.

Adds HasAllRelevantServices and GetRelevantServices, which check
for NETWORK|WITNESS.

This changes the following:
 * Removes nRelevantServices from CConnman, disconnecting it a bit
   more from protocol-level logic.
 * Replaces our sometimes-connect-to-!WITNESS-nodes logic with
   simply always requiring WITNESS|NETWORK for outbound non-feeler
   connections (feelers still only require NETWORK).
 * This has the added benefit of removing nServicesExpected from
   CNode - instead letting net_processing's VERSION message
   handling simply check HasAllRelevantServices.
 * This implies we believe WITNESS nodes to continue to be a
   significant majority of nodes on the network, but also because
   we cannot sync properly from !WITNESS nodes, it is strange to
   continue using our valuable outbound slots on them.
 * In order to prevent this change from preventing connection to
   -connect= nodes which have !WITNESS, -connect nodes are now
   given the "addnode" flag. This also allows outbound connections
   to !NODE_NETWORK nodes for -connect nodes (which was already true
   of addnodes).
 * Has the (somewhat unintended) consequence of changing one of the
   eviction metrics from the same
   sometimes-connect-to-!WITNESS-nodes metric to requiring
   HasRelevantServices.

This should make NODE_NETWORK_LIMITED much simpler to implement.

src/protocol.h

@@ -277,6 +277,43 @@ enum ServiceFlags : uint64_t {
     // BIP process.
 };
 
+/**
+ * Gets the set of service flags which are "desirable" for a given peer.
+ *
+ * These are the flags which are required for a peer to support for them
+ * to be "interesting" to us, ie for us to wish to use one of our few
+ * outbound connection slots for or for us to wish to prioritize keeping
+ * their connection around.
+ *
+ * Relevant service flags may be peer- and state-specific in that the
+ * version of the peer may determine which flags are required (eg in the
+ * case of NODE_NETWORK_LIMITED where we seek out NODE_NETWORK peers
+ * unless they set NODE_NETWORK_LIMITED and we are out of IBD, in which
+ * case NODE_NETWORK_LIMITED suffices).
+ *
+ * Thus, generally, avoid calling with peerServices == NODE_NONE.
+ */
+static ServiceFlags GetDesirableServiceFlags(ServiceFlags services) {
+    return ServiceFlags(NODE_NETWORK | NODE_WITNESS);
+}
+
+/**
+ * A shortcut for (services & GetDesirableServiceFlags(services))
+ * == GetDesirableServiceFlags(services), ie determines whether the given
+ * set of service flags are sufficient for a peer to be "relevant".
+ */
+static inline bool HasAllDesirableServiceFlags(ServiceFlags services) {
+    return !(GetDesirableServiceFlags(services) & (~services));
+}
+
+/**
+ * Checks if a peer with the given service flags may be capable of having a
+ * robust address-storage DB. Currently an alias for checking NODE_NETWORK.
+ */
+static inline bool MayHaveUsefulAddressDB(ServiceFlags services) {
+    return services & NODE_NETWORK;
+}
+
 /** A CService with information about it as peer */
 class CAddress : public CService
 {