Profiling: Add base, fix memory profiling (#8850)

For 'snapshot' type profiles, record a 'before' profile that can be used 
as `go tool pprof -base=before ...` to compare before and after.

"Before" profiles are included in the zipped package.

[`runtime.MemProfileRate`](https://golang.org/pkg/runtime/#pkg-variables) 
should not be updated while the application is running, so we set it at startup.

Co-authored-by: Harshavardhana <harsha@minio.io>

cmd/signals.go

@@ -30,10 +30,8 @@ func handleSignals() {
 		// If global profiler is set stop before we exit.
 		globalProfilerMu.Lock()
 		defer globalProfilerMu.Unlock()
-		if len(globalProfiler) > 0 {
-			for _, p := range globalProfiler {
-				p.Stop()
-			}
+		for _, p := range globalProfiler {
+			p.Stop()
 		}
 
 		if success {