main: Add prune flag

This change is part of the effort to add pruning support to btcd.

Pruning is now available to the end user via --prune flag.  There are
checks in place so that the user doesn't go below the minimum prune
target of 1536 MiB.  The minimum is set so that we keep at least 288
blocks per the requirement for NODE_NETWORK_LIMITED nodes specified by
BIP0159.  The default value of 0 will disable pruning.

config.go

@@ -66,6 +66,7 @@ const (
 	sampleConfigFilename         = "sample-btcd.conf"
 	defaultTxIndex               = false
 	defaultAddrIndex             = false
+	pruneMinSize                 = 1536
 )
 
 var (
@@ -146,6 +147,7 @@ type config struct {
 	Proxy                string        `long:"proxy" description:"Connect via SOCKS5 proxy (eg. 127.0.0.1:9050)"`
 	ProxyPass            string        `long:"proxypass" default-mask:"-" description:"Password for proxy server"`
 	ProxyUser            string        `long:"proxyuser" description:"Username for proxy server"`
+	Prune                uint64        `long:"prune" description:"Prune already validated blocks from the database. Must specify a target size in MiB (minimum value of 1536, default value of 0 will disable pruning)"`
 	RegressionTest       bool          `long:"regtest" description:"Use the regression test network"`
 	RejectNonStd         bool          `long:"rejectnonstd" description:"Reject non-standard transactions regardless of the default settings for the active network."`
 	RejectReplacement    bool          `long:"rejectreplacement" description:"Reject transactions that attempt to replace existing transactions within the mempool through the Replace-By-Fee (RBF) signaling policy."`
@@ -1137,6 +1139,14 @@ func loadConfig() (*config, []string, error) {
 		}
 	}
 
+	if cfg.Prune != 0 && cfg.Prune < pruneMinSize {
+		err := fmt.Errorf("%s: the minimum value for --prune is %d. Got %d",
+			funcName, pruneMinSize, cfg.Prune)
+		fmt.Fprintln(os.Stderr, err)
+		fmt.Fprintln(os.Stderr, usageMessage)
+		return nil, nil, err
+	}
+
 	// Warn about missing config file only after all other configuration is
 	// done.  This prevents the warning on help messages and invalid
 	// options.  Note this should go directly before the return.

server.go

@@ -2730,6 +2730,9 @@ func newServer(listenAddrs, agentBlacklist, agentWhitelist []string,
 	if cfg.NoCFilters {
 		services &^= wire.SFNodeCF
 	}
+	if cfg.Prune != 0 {
+		services &^= wire.SFNodeNetwork
+	}
 
 	amgr := addrmgr.New(cfg.DataDir, btcdLookup)
 
@@ -2831,6 +2834,7 @@ func newServer(listenAddrs, agentBlacklist, agentWhitelist []string,
 		SigCache:     s.sigCache,
 		IndexManager: indexManager,
 		HashCache:    s.hashCache,
+		Prune:        cfg.Prune * 1024 * 1024,
 	})
 	if err != nil {
 		return nil, err