Convert more peer <--> peer REST calls (#19004)

* Convert more peer <--> peer REST calls
* Clean up in general.
* Add JSON wrapper.
* Add slice wrapper.
* Add option to make handler return nil error if no connection is given, `IgnoreNilConn`.

Converts the following:

```
+	HandlerGetMetrics
+	HandlerGetResourceMetrics
+	HandlerGetMemInfo
+	HandlerGetProcInfo
+	HandlerGetOSInfo
+	HandlerGetPartitions
+	HandlerGetNetInfo
+	HandlerGetCPUs
+	HandlerServerInfo
+	HandlerGetSysConfig
+	HandlerGetSysServices
+	HandlerGetSysErrors
+	HandlerGetAllBucketStats
+	HandlerGetBucketStats
+	HandlerGetSRMetrics
+	HandlerGetPeerMetrics
+	HandlerGetMetacacheListing
+	HandlerUpdateMetacacheListing
+	HandlerGetPeerBucketMetrics
+	HandlerStorageInfo
+	HandlerGetLocks
+	HandlerBackgroundHealStatus
+	HandlerGetLastDayTierStats
+	HandlerSignalService
+	HandlerGetBandwidth
```

cmd/local-locker.go

@@ -17,6 +17,8 @@
 
 package cmd
 
+//go:generate msgp -file=$GOFILE -unexported
+
 import (
 	"context"
 	"fmt"
@@ -36,12 +38,9 @@ type lockRequesterInfo struct {
 	TimeLastRefresh time.Time // Timestamp for last lock refresh.
 	Source          string    // Contains line, function and filename requesting the lock.
 	Group           bool      // indicates if it was a group lock.
-	// Owner represents the UUID of the owner who originally requested the lock
-	// useful in expiry.
-	Owner string
-	// Quorum represents the quorum required for this lock to be active.
-	Quorum int
-	idx    int
+	Owner           string    // Owner represents the UUID of the owner who originally requested the lock.
+	Quorum          int       // Quorum represents the quorum required for this lock to be active.
+	idx             int       `msg:"-"` // index of the lock in the lockMap.
 }
 
 // isWriteLock returns whether the lock is a write or read lock.
@@ -50,6 +49,8 @@ func isWriteLock(lri []lockRequesterInfo) bool {
 }
 
 // localLocker implements Dsync.NetLocker
+//
+//msgp:ignore localLocker
 type localLocker struct {
 	mutex   sync.Mutex
 	lockMap map[string][]lockRequesterInfo
@@ -238,7 +239,9 @@ func (l *localLocker) stats() lockStats {
 	return st
 }
 
-func (l *localLocker) DupLockMap() map[string][]lockRequesterInfo {
+type localLockMap map[string][]lockRequesterInfo
+
+func (l *localLocker) DupLockMap() localLockMap {
 	l.mutex.Lock()
 	defer l.mutex.Unlock()