ListObjects Metadata Caching (#10648)

Design: https://gist.github.com/klauspost/025c09b48ed4a1293c917cecfabdf21c

Gist of improvements:

* Cross-server caching and listing will use the same data across servers and requests.
* Lists can be arbitrarily resumed at a constant speed.
* Metadata for all files scanned is stored for streaming retrieval.
* The existing bloom filters controlled by the crawler is used for validating caches.
* Concurrent requests for the same data (or parts of it) will not spawn additional walkers.
* Listing a subdirectory of an existing recursive cache will use the cache.
* All listing operations are fully streamable so the number of objects in a bucket no 
  longer dictates the amount of memory.
* Listings can be handled by any server within the cluster.
* Caches are cleaned up when out of date or superseded by a more recent one.

cmd/bucket-handlers_test.go

@@ -19,7 +19,6 @@ package cmd
 import (
 	"bytes"
 	"encoding/xml"
-	"fmt"
 	"io/ioutil"
 	"net/http"
 	"net/http/httptest"
@@ -837,7 +836,7 @@ func testAPIDeleteMultipleObjectsHandler(obj ObjectLayer, instanceType, bucketNa
 
 		// Verify whether the bucket obtained object is same as the one created.
 		if testCase.expectedContent != nil && !bytes.Equal(testCase.expectedContent, actualContent) {
-			fmt.Println(string(testCase.expectedContent), string(actualContent))
+			t.Log(string(testCase.expectedContent), string(actualContent))
 			t.Errorf("Test %d : MinIO %s: Object content differs from expected value.", i+1, instanceType)
 		}
 	}