Implement reminder provider

[suraciii]

This PR implemented reminder provider based on Redis.

As a reminder table implementation, it needs to support various queries for reminders:

1. query a single reminder by its grain and name.
2. query reminders by its grain.
3. query reminders by a range of grain hash.
4. query a single reminder by its grain, name and eTag (for removal).

In this implementation, all reminders are stored in a single SortedSet in Redis, and the data of reminder is formatted as serialized JSON array like this:
["$grainHash","$grainRef.ToKeyString()","$reminderName","$eTag","$startAt.ToString("O")","$period.ToString()"]

So that reminders can be queried by following filters:

1. ["$grainHash","$grainRef.ToKeyString()","$reminderName","
2. ["$grainHash","$grainRef.ToKeyString()","
3. ["$grainHashStart"," and ["$grainHashEnd","
4. ["$grainHash","$grainRef.ToKeyString()","$reminderName","$eTag","

Unfortunately, Redis doesn't support multi-indexes natively, so the data need to be stored in various places to support various queries:
1. in HashSet: $"{serviceId}_{grainRef.ToKeyString()}_{reminderName}", it has two fields ETag and Data, one HashSet only has one reminder's data.
2. in HashSet: $"{serviceId}_{grainRef.ToKeyString()}", it stores all reminders of a grain, reminder names are set as field keys, so that we can query reminders by a grain.
3. in SortedSet: $"{serviceId}_GrainHashIndex", it stores all reminders with grain hashes as scores, so that we can query reminders by a range of grain hash.

[ReubenBond]

Could this be done using a single SortedSet, queried by ZRANGEBYLEX?
The keys, in that case, could take the form: $"{serviceId}_{hashCode:X8}_{grainRef.ToKeyString()}_{reminderName}"

I think your approach is fine, too. Redis is not designed for scalability.

[suraciii]

@ReubenBond I updated it with using a single SortedSet to do this.
Reminders save in the SortedSet with the pattern below:

$"{hashCode:X8}_{grainRef}_{reminderName}:{eTag}:{new {entry.StartAt, entry.Period}}"

But this pattern is quite fragile since any ':' or '_' in reminder name or string-typed grain id will break it. I think using a json array for this may be a better idea, I'll try it later

[ReubenBond]

@suraciii, please let me know when you feel this is ready for final review

[suraciii]

Hi @ReubenBond , It's ready for review now