Refactor redis keys and store messages in protobuf

Changes:
- Task messages are stored under "asynq:{<qname>}:t:<task_id>" key in redis, value is a HASH type and message are stored under "msg" key in the hash. The hash also stores "deadline", "timeout".
- Redis LIST and ZSET stores task message IDs
- Task messages are serialized using protocol buffer

internal/asynqtest/asynqtest.go

@@ -6,7 +6,6 @@
 package asynqtest
 
 import (
-	"encoding/json"
 	"math"
 	"sort"
 	"testing"