Fix flaky TestCacheDel test

[jarifibrahim]

The TestCacheDel test inserts an item and deletes it. The inserts and deletes are pushed through the setBuf channel.
The test would fail if the Get was done before the previous delete was performed. This test adds a sleep to ensure the Get happens only after the delete is done.

The failure can be reproduced locally by applying the following diff.

diff --git a/cache.go b/cache.go
index e48a48a..42a381f 100644
--- a/cache.go
+++ b/cache.go
@@ -247,6 +247,10 @@ func (c *Cache) Del(key interface{}) {
 	keyHash, conflictHash := c.keyToHash(key)
 	// Delete immediately.
 	c.store.Del(keyHash, conflictHash)
+
+	// This will fail the TestCacheDel test.
+	time.Sleep(time.Millisecond)
+
 	// If we've set an item, it would be applied slightly later.
 	// So we must push the same item to `setBuf` with the deletion flag.
 	// This ensures that if a set is followed by a delete, it will be

An example of the flaky crash https://travis-ci.org/github/dgraph-io/ristretto/jobs/703421284#L246

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This change is 

[martinmr]

Reviewed 1 of 1 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @anurags92, @karlmcguire, and @manishrjain)