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/storage-datatypes.go

@@ -21,6 +21,8 @@ import (
 	"time"
 )
 
+//go:generate msgp -file=$GOFILE
+
 // DeleteOptions represents the disk level delete options available for the APIs
 type DeleteOptions struct {
 	BaseOptions
@@ -44,8 +46,6 @@ type DiskInfoOptions struct {
 	NoOp    bool   `msg:"np"`
 }
 
-//go:generate msgp -file=$GOFILE
-
 // DiskInfo is an extended type which returns current
 // disk usage per path.
 // The above means that any added/deleted fields are incompatible.
@@ -445,3 +445,8 @@ type RenameDataResp struct {
 type LocalDiskIDs struct {
 	IDs []string
 }
+
+// ListDirResult - ListDir()'s response.
+type ListDirResult struct {
+	Entries []string `msg:"e"`
+}