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.

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