fix list blocking operations

[meteorgan]

about list blocking operations, still have some problems:

1. not support multi keys. supporting multi keys in multi keyNode needs much more works.
2. the scheduler in the block function may has race conditions with unblock. Is sending an unblock message in the scheduler a better solution?

[meteorgan]

@stephenmcd do you have any advice for this PR?

[stephenmcd]

Sorry I didn't have time to look at it yet, the other recent changes were very quick to review.

It might save some time if you can provide a set of commands or a script that reproduces the problem.

[meteorgan]

OK, I will do it at this night.

[meteorgan]

1. blpop only blocks 1s no matter how much timeout is setted. Executing blocking operations should not set timeout.
   redis 127.0.0.1:6370> lrange list2 0 -1
   (empty list or set)
   redis 127.0.0.1:6370> blpop list2 5
   (error) ERR Timeout on blpop list2 5
   (1.01s)
2. this is because in unblock change command to the former block command, but does not change back,so there is no response back.
   a.
   redis 127.0.0.1:6370> blpop list2 30

1. "list2"
2. "xxx"
   (6.98s)
   b. block forever
   redis 127.0.0.1:6370> lpush list2 xxx

3. timeout 0 does not work.
   redis 127.0.0.1:6370> blpop list2 0
   (nil)
4. scheduler setted in block function is not cancelled in unblock function.