Support TTL of member

[caipengbo]

Background

Kvrocks currently supports TTL(Time to Live) at the key level. In some scenarios, Users need members to support TTL.

#283 also raises this issue. As @git-hulk said in this issue, the implementation is not particularly difficult, but the changes may be quite large because we should add new type and there are so many additional commands to support.

Here are my design and implementation ideas that I would like to discuss with developers. I take Hash as an example:

Design

1. Add a new type, called ExHash
2. Based on the coding design of Hash Type, we encode expire time in the value of subkey:

        +----------+------------+-----------+-----------+
key =>  |  flags   |  expire    |  version  |  size     |
        | (1byte)  | (4byte)    |  (8byte)  | (8byte)   |
        +----------+------------+-----------+-----------+

                     +---------------+
key|version|field => |     value     |
                     |     (Nbyte)   |
                     +---------------+

                     +----------+---------------+
key|version|field => |  expire  |   value       |
                     |  (4byte) |  (Nbyte)      |
                     +----------+---------------+

3. In addition to commands that have the same semantics as Hash, ExHash has the following special commands:

Command	Syntax	Description
EXHSETEX	EXHSETEX key field value seconds	Adds a field with a second timeout to a specified ExHash. If the key does not exist, a key for the ExHash is created. If the field already existed, this command overwrites the value of the field.
EXHEXPIRE	EXHEXPIRE key field seconds	Specifies the timeout of a field in a specified ExHash. If the field timeout already existed, this command overwrites the timeout of the field.
EXHEXPIREAT	EXHEXPIREAT key field timestamp	Like EXHEXPIRE, just using unix timestamp.
EXHTTL	EXHTTL key field	Get the remaining expiration time of a field in a specified ExHash. Unit: Seconds.
EXHPERSIST	EXHPERSIST key field	Remove the timeout of a field in a specified ExHash.

Implement

1. Add a new RedisType, called kRedisExHash
2. Modify the InternalKey class and add member variables about TTL to it. When users obtain corresponding values, they need to check expire
3. In SubKeyFilter::IsKeyExpired, judge whether the Value expire

[caipengbo]

How do you think @KvrocksLabs/committers ?

[git-hulk]

Many thanks for @caipengbo starting the discussion and draft design here.

As mentioned previously in our slack channel, the only issue was that accessing the metadata size field would have the data race in the compaction filter. I and @ShooterIT agree that can sacrifice the hlen command. I'm not worried about the huge change if our design was simple enough, also we have many eyes to make bugs shallow.

cc @ShooterIT @Alfejik @smartlee @popunit