connmgr: Refactor connection management into pkg

This commit introduces package connmgr which contains connection
management related functionality.

The following is an overview of the features the package provides:

- Maintain fixed number of outbound connections
- Optional connect-only mode
- Retry persistent connections with increasing back-off
- Source peers from DNS seeds
- Use Tor to resolve DNS
- Dynamic ban scores
- Test coverage

In addition, btcd has been refactored to make use of the new package by
extending the connection manager to work with the server to source and
maintain peer connections. The following is a broad overview of the
changes to integrate the package:

- Simplify peer state by removing pending, retry peers
- Refactor to remove retries which are now handled by connmgr
- Use callback to add addresses sourced from the  DNS seed

Finally the following connection-related things have been improved as a
part of this refactor:

- Fixes 100% cpu usage when network is down (#129)
- Fixes issues with max peers (#577)
- Simplify outbound peer connections management

config.go

@@ -20,6 +20,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/btcsuite/btcd/connmgr"
 	"github.com/btcsuite/btcd/database"
 	_ "github.com/btcsuite/btcd/database/ffldb"
 	"github.com/btcsuite/btcd/mempool"
@@ -860,7 +861,7 @@ func loadConfig() (*config, []string, error) {
 		cfg.dial = proxy.Dial
 		if !cfg.NoOnion {
 			cfg.lookup = func(host string) ([]net.IP, error) {
-				return torLookupIP(host, cfg.Proxy)
+				return connmgr.TorLookupIP(host, cfg.Proxy)
 			}
 		}
 	}
@@ -900,7 +901,7 @@ func loadConfig() (*config, []string, error) {
 			return proxy.Dial(a, b)
 		}
 		cfg.onionlookup = func(host string) ([]net.IP, error) {
-			return torLookupIP(host, cfg.OnionProxy)
+			return connmgr.TorLookupIP(host, cfg.OnionProxy)
 		}
 	} else {
 		cfg.oniondial = cfg.dial

connmgr/README.md

@@ -0,0 +1,39 @@
+connmgr
+=======
+
+[![Build Status](http://img.shields.io/travis/btcsuite/btcd.svg)]
+(https://travis-ci.org/btcsuite/btcd) [![ISC License]
+(http://img.shields.io/badge/license-ISC-blue.svg)](http://copyfree.org)
+[![GoDoc](https://img.shields.io/badge/godoc-reference-blue.svg)]
+(http://godoc.org/github.com/btcsuite/btcd/connmgr)
+
+Package connmgr implements a generic Bitcoin network connection manager.
+
+## Overview
+
+Connection Manager handles all the general connection concerns such as
+maintaining a set number of outbound connections, sourcing peers, banning,
+limiting max connections, tor lookup, etc.
+
+The package provides a generic connection manager which is able to accept
+connection requests from a source or a set of given addresses, dial them and
+notify the caller on connections. The main intended use is to initialize a pool
+of active connections and maintain them to remain connected to the P2P network.
+
+In addition the connection manager provides the following utilities:
+
+- Notifications on connections or disconnections
+- Handle failures and retry new addresses from the source
+- Connect only to specified addresses
+- Permanent connections with increasing backoff retry timers
+- Disconnect or Remove an established connection
+
+## Installation and Updating
+
+```bash
+$ go get -u github.com/btcsuite/btcd/connmgr
+```
+
+## License
+
+Package connmgr is licensed under the [copyfree](http://copyfree.org) ISC License.