lib/config, lib/connections: Add optional connection limits (fixes #7176) (#7223)

This adds two new configuration options:

    // The number of connections at which we stop trying to connect to more
    // devices, zero meaning no limit. Does not affect incoming connections.
    ConnectionLimitEnough int

    // The maximum number of connections which we will allow in total, zero
    // meaning no limit. Affects incoming connections and prevents
    // attempting outgoing connections.
    ConnectionLimitMax int

These can be used to limit the number of concurrent connections in
various ways.

go.mod

@@ -17,7 +17,6 @@ require (
 	github.com/gobwas/glob v0.2.3
 	github.com/gogo/protobuf v1.3.1
 	github.com/golang/groupcache v0.0.0-20200121045136-8c9f03a8e57e
-	github.com/golang/protobuf v1.4.3
 	github.com/greatroar/blobloom v0.5.0
 	github.com/jackpal/gateway v1.0.6
 	github.com/jackpal/go-nat-pmp v1.0.2
@@ -46,6 +45,7 @@ require (
 	golang.org/x/sys v0.0.0-20201101102859-da207088b7d1
 	golang.org/x/text v0.3.4
 	golang.org/x/time v0.0.0-20200630173020-3af7569d3a1e
+	google.golang.org/protobuf v1.23.0
 )
 
 go 1.14