Add ExHash type, support field expire

[caipengbo]

Background

I opened a Discussion #399 in which we discussed support TTL for member and I proposed a draft design. Based on this design, I extended the Hash type to support field expiration.

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:

metadata:

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

subkey:

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

Encode expire in subkey's value:

                     +----------+---------------+
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. Returns OK on success. When the seconds parameter is invalid, the command returns an error.
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. Returns 1 on success or 0 if key or field does not exist. When the seconds parameter is invalid, the command returns an error.
EXHEXPIREAT	EXHEXPIREAT key field timestamp	Like EXHEXPIRE, just using unix timestamp. Returns 1 on success or 0 if key or field does not exist. The command returns an error when the timestamp parameter is invalid. Note: If timestamp is less than the current timestamp, the field will not set the lifetime.
EXHTTL	EXHTTL key field	Get the remaining expiration time of a field in a specified ExHash. Unit: Seconds.
EXHPERSIST	EXHPERSIST key field	Remove the expire of a field in a specified ExHash. Returns 1 when the expire is successfully removed. Returns 0 if field has no expiration set or if ExHash or field does not exist.
EXHLEN	EXHLEN key [EXACT]	Get the number of fields about a specified ExHash. If there is no EXACT flag, the length returned is inaccurate (it will contain expired fields that have not been reclaimed). If EXACT flag is set, the exact length is obtained, but this is time-consuming because it will check the entire ExHash. Return the number of fields in a ExHash. Returns 0 if key does not exist.
EXHGETALL	EXHGETALL key [WITHTTL]	Returns all fields and values in the ExHash table key without the optional WITHTTL parameter. When WITHTTL is added, all fields, values, and lifetime in the ExHash key are returned. Returns the ExHash's fields and their values as a list, along with the ttl (-1 if no expire is set). If the key does not exist, the empty list is returned.

The other commands are the same as Hash. In particular, I didn't implement EXHMSET (synonymous with HMSET) because this command can be used instead of EXHSET (synonymous with HSET) to set multiple fields and values.

Implement

1. Add a new RedisType, called kRedisExHash
2. Modify the Hash class and add member variables support_field_expire_ to determine whether field expiration is supported. When get the value of the field, we need to check expire.
3. In SubKeyFilter::IsKeyExpired, judge whether the Value expire. Note that we do not update the size in metadata because this accesses the Storage object and causes data race, which will affect performance. So the size in ExHash metadata is inaccurate, and it contains fields that have expired.

[caipengbo]

Can deal with part of the #283

[ColinChamber]

Cool. I have some suggestions, maybe it would be better to be more similar to the interface of Tairhash. Many people have already used it, which already been provided to a client.

[caipengbo]

Thank you very much. I will take your advice seriously. @ColinChamber

[tisonkun]

Closed as stale.

Codebase evolves a lot since the last activities on this patch. There're a few of conflicts. @caipengbo you're welcome to propose the changes again based on the nightly unstable branch.