Add Profiler Admin API (#6463)

Two handlers are added to admin API to enable profiling and disable
profiling of a server in a standalone mode, or all nodes in the
distributed mode.

/minio/admin/profiling/start/{cpu,block,mem}:
  - Start profiling and return starting JSON results, e.g. one
    node is offline.

/minio/admin/profiling/download:
  - Stop the on-going profiling task
  - Stream a zip file which contains all profiling files that can
    be later inspected by go tool pprof

vendor/github.com/pkg/profile/trace16.go

@@ -0,0 +1,10 @@
+// +build !go1.7
+
+package profile
+
+import "io"
+
+// mock trace support for Go 1.6 and earlier.
+
+func startTrace(w io.Writer) error { return nil }
+func stopTrace()                   {}