Set correct cluster slot for scan commands, similarly to Java's Jedis client (#2623)

- At present, the `scan` command is dispatched to a random slot.
- As far as I can tell, the scanX family of commands are not cluster aware (e.g. don't redirect the client to the correct slot).
- You can see [here](869dc0bb66/src/main/java/redis/clients/jedis/ShardedCommandObjects.java (L101)), the Jedis client calling `processKey` on the match argument, and this is what this PR also does.

We've had this patch running in production, and it seems to work well for us.

For further thought:
- Continuing looking at other Redis clients (e.g. Jedis), they outright [reject as invalid](869dc0bb66/src/main/java/redis/clients/jedis/ShardedCommandObjects.java (L98)) any scan command that does not include a hash-tag. Presumably this has the advantage of users not being surprised when their scan produces no results when a random server is picked.
- Perhaps it would be sensible for go-redis to do the same also?

Co-authored-by: Nedyalko Dyakov <1547186+ndyakov@users.noreply.github.com>

internal/hashtag/hashtag.go

@@ -56,6 +56,18 @@ func Key(key string) string {
 	return key
 }
 
+func Present(key string) bool {
+	if key == "" {
+		return false
+	}
+	if s := strings.IndexByte(key, '{'); s > -1 {
+		if e := strings.IndexByte(key[s+1:], '}'); e > 0 {
+			return true
+		}
+	}
+	return false
+}
+
 func RandomSlot() int {
 	return rand.Intn(slotNumber)
 }

internal/hashtag/hashtag_test.go

@@ -69,3 +69,28 @@ var _ = Describe("HashSlot", func() {
 		}
 	})
 })
+
+var _ = Describe("Present", func() {
+	It("should calculate hash slots", func() {
+		tests := []struct {
+			key     string
+			present bool
+		}{
+			{"123456789", false},
+			{"{}foo", false},
+			{"foo{}", false},
+			{"foo{}{bar}", false},
+			{"", false},
+			{string([]byte{83, 153, 134, 118, 229, 214, 244, 75, 140, 37, 215, 215}), false},
+			{"foo{bar}", true},
+			{"{foo}bar", true},
+			{"{user1000}.following", true},
+			{"foo{{bar}}zap", true},
+			{"foo{bar}{zap}", true},
+		}
+
+		for _, test := range tests {
+			Expect(Present(test.key)).To(Equal(test.present), "for %s", test.key)
+		}
+	})
+})