Merge #14929: net: Allow connections from misbehavior banned peers

0297be61a Allow connections from misbehavior banned peers. (Gregory Maxwell)

Pull request description:

  This allows incoming connections from peers which are only banned
   due to an automatic misbehavior ban if doing so won't fill inbound.

  These peers are preferred for eviction when inbound fills, but may
   still be kept if they fall into the protected classes.  This
   eviction preference lasts the entire life of the connection even
   if the ban expires.

  If they misbehave again they'll still get disconnected.

  The main purpose of banning on misbehavior is to prevent our
   connections from being wasted on unhelpful peers such as ones
   running incompatible consensus rules.  For inbound peers this
   can be better accomplished with eviction preferences.

  A secondary purpose was to reduce resource waste from repeated
   abuse but virtually any attacker can get a nearly unlimited
   supply of addresses, so disconnection is about the best we can
   do.

  This can reduce the potential from negative impact due to incorrect misbehaviour bans.

Tree-SHA512: 03bc8ec8bae365cc437daf70000c8f2edc512e37db821bc4e0fafa6cf56cc185e9ab40453aa02445f48d6a2e3e7268767ca2017655aca5383108416f1e2cf20f

src/net.h

@@ -651,6 +651,7 @@ public:
     // the network or wire types and the cleaned string used when displayed or logged.
     std::string strSubVer GUARDED_BY(cs_SubVer), cleanSubVer GUARDED_BY(cs_SubVer);
     CCriticalSection cs_SubVer; // used for both cleanSubVer and strSubVer
+    bool m_prefer_evict{false}; // This peer is preferred for eviction.
     bool fWhitelisted{false}; // This peer can bypass DoS banning.
     bool fFeeler{false}; // If true this node is being used as a short lived feeler.
     bool fOneShot{false};