add scheduler client (with caching)

[cgardens]

This is a low priority PR. Wanted to try something out on caching while I was making changes in the area. This PR does not block anything related to your current weekly or monthly goals

What

• @michel-tricot pointed out that we could do a better job of hiding the caching in: add spec cache #1066. Took another crack at it here.

How

• remove spec cache
• replace it with a scheduler client interface.
• put caching behind the client interface.

Outcome

• I am happier with the structure of the code.
• I still feel like handling cache invalidation isn't maybe as hidden as you had hoped @michel-tricot . Assuming that we need this invalidation, curious if you see something to do better here. We can also just forget about it and have someone restart the server to invalidate the cache.
• Definitely took 3x longer to implement than the original spec cache. 😛 . It's better and worth it.

Checklist

• Unit Tests for client

Recommended reading order

1. SchedulerJobClient
2. DefaultSchedulerJobClient
3. CachingSchedulerJobClient.
4. SpecCachingSchedulerJobClient.
5. SchedulerHandler
6. the rest

[cgardens]

this is the part i mentioned in the PR description. caching isn't fully hidden. but couldn't really think of another way to allow the caller to invalidate the cache without exposing it like this.

[michel-tricot]

I usually delegation instead of inheritance for that kind of class:

new SpecCacheSJC(client);

most of the methods would just call:
client.xxx()

and only the one where you want to change behavior you add your code.

That allows you to compose features on top of the DefaultClient

You can argue that this is over-engineered.

[michel-tricot]

Also since you can not hide the cache invalidation, I guess it doesn't really matter.