Peer preferentially with NODE_CASH nodes

Summary:
Add NODE_BITCOIN_CASH to required services for outbound
peer connections.
Not required for inbound peers.

Test Plan:
- Remove peers.dat
- Start up node
- Monitor peer connections using e.g.
    $ bitcoin-cli getpeerinfo | egrep '(service|subver|inbound)'
- Check that all peers for which inbound is false have bit 5
  (0x20) set in their reported services
- Connect to various other types of peers (BU, Classic, XT)
  using `addnode` and check that connections are successful
- Check that inbound peers without NODE_CASH service bit set
  can connect (you should get occasional crawlers or Satoshi
  clients connecting with services < 0x20)

Reviewers: #bitcoin_abc, deadalnix, CCulianu

Reviewed By: #bitcoin_abc, CCulianu

Subscribers: CCulianu

Differential Revision: https://reviews.bitcoinabc.org/D457

src/init.cpp

@@ -1520,6 +1520,9 @@ bool AppInitParameterInteraction(Config &config) {
     // to differentiate the network nodes.
     nLocalServices = ServiceFlags(nLocalServices | NODE_BITCOIN_CASH);
 
+    // Preferentially keep peers which service NODE_BITCOIN_CASH
+    nRelevantServices = ServiceFlags(nRelevantServices | NODE_BITCOIN_CASH);
+
     nMaxTipAge = GetArg("-maxtipage", DEFAULT_MAX_TIP_AGE);
 
     if (mapMultiArgs.count("-bip9params")) {

src/net.h

@@ -94,7 +94,8 @@ static const bool DEFAULT_FORCEDNSSEED = true;
 static const size_t DEFAULT_MAXRECEIVEBUFFER = 5 * 1000;
 static const size_t DEFAULT_MAXSENDBUFFER = 1 * 1000;
 
-static const ServiceFlags REQUIRED_SERVICES = NODE_NETWORK;
+static const ServiceFlags REQUIRED_SERVICES =
+    ServiceFlags(NODE_NETWORK | NODE_BITCOIN_CASH);
 
 // Default 24-hour ban.
 // NOTE: When adjusting this, update rpcnet:setban's help ("24h")