Add GetAbsExpire/SetAbsExpire for module

[chenyang8094]

Similar to the SET key value [PXAT <milliseconds-timestamp>], it is often necessary to set and get the absolute expire time of the key in the module. Otherwise I can only do it by this:

long long milliseconds = expire - RedisModule_Milliseconds();  
RedisModule_SetExpire(key, milliseconds);

Now i can do it by this:

RedisModule_SetAbsExpire(key, expire);

At the same time, in the previous implementation of RM_SetExpire, expire is not forced to be a positive value, so it is possible to get a value that conflicts with REDISMODULE_NO_EXPIRE(-1) in RM_GetExpire, so I Add a check to ensure that the expire parameters in RM_SetExpire and RM_SetAbsExpire must be positive.

[soloestoy]

I think the new APIs are useful, ping @redis/core-team get more suggestions.

[oranagra]

i agree with the usefulness of this API.
i have some concern about the details, but on further thinking i think the current version may be ok.