fix: fix bug in del function of redis_proxy

[levy5307]

What problem does this PR solve?

In redis_proxy，"OK" will be returned using del function to delete a key, If the function executes successfully. But interface definition of redis shows that it should return the number of keys which are deleted.
While, there are some difference between interface definition of redis and redis proxy of pegasus. Because whether the key is exist or not, pegasus will return "OK" to redis proxy, so we cann't distinguish whether the key is deleted or not. so in this case , we unity returns 1.

Redis documentation about del: https://redis.io/commands/del

DEL key [key ...]
Removes the specified keys. A key is ignored if it does not exist.
Return value
Integer reply: The number of keys that were removed.
Examples

redis> SET key1 "Hello"
"OK"
redis> SET key2 "World"
"OK"
redis> DEL key1 key2 key3
(integer) 2
redis> 

What is changed and how it works?

Change the return value from "OK" to the number of keys.

Check List

Tests

• Manual test (add detailed scripts or steps below)
  1.start a redis proxy in c4tst-injection
  2.clone redis code, and build it.
  3.use command of ./redis_cli -h host -p port to connect with the redis proxy
  4.insert a k-v pair into redis proxy:

redis> SET key "value"
"OK"

5. in pervious versionm, regardless of the key is exist or not, del the key, it returns "OK"

redis> DEL key "value"
"OK"

6.del the key which was inserted before in current version. It returns 1, not "OK".

redis> DEL key "value"
(integer) 1

7.del a key which is not exist in current version. It returns 1, also.

redis> DEL not_exist_key "value"
(integer) 1

Related changes

• Need to cherry-pick to the release branch

[neverchanje]

Can you include the manual test in the PR description? This PR does not show any kind of test for us.

Like this:

Previous version

> redis del a
> ok

Current version

> redis del a a
> -1
> redis del a
> 1

[neverchanje]

I tried these commands on http://try.redis.io/. Deleting a non-existed key will return 0, but your version returns 1. This should be noted in our code.

> set a a
OK
> del a
(integer) 1
> del a
(integer) 0