Allow connections from misbehavior banned peers.

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.

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};