Fix blocking operations from missing new lists

Behrad Zari discovered [1] and Josiah reported [2]: if you block
and wait for a list to exist, but the list creates from
a non-push command, the blocked client never gets notified.

This commit adds notification of blocked clients into
the DB layer and away from individual commands.

Lists can be created by [LR]PUSH, SORT..STORE, RENAME, MOVE,
and RESTORE.  Previously, blocked client notifications were
only triggered by [LR]PUSH.  Your client would never get
notified if a list were created by SORT..STORE or RENAME or
a RESTORE, etc.

Blocked client notification now happens in one unified place:
  - dbAdd() triggers notification when adding a list to the DB

Two new tests are added that fail prior to this commit.

All test pass.

Fixes #1668

[1]: https://groups.google.com/forum/#!topic/redis-db/k4oWfMkN1NU
[2]: #1668

src/redis.h

@@ -1037,6 +1037,7 @@ void listTypeConvert(robj *subject, int enc);
 void unblockClientWaitingData(redisClient *c);
 void handleClientsBlockedOnLists(void);
 void popGenericCommand(redisClient *c, int where);
+void signalListAsReady(redisDb *db, robj *key);
 
 /* MULTI/EXEC/WATCH... */
 void unwatchAllKeys(redisClient *c);