chore: simplify reward distribution code

[mfontanini]

This simplifies the reward distribution since it was pretty convoluted and hard to follow. Specifically this:

• Avoids sharing the reward data so instead of living in the KeeperState it lives inside the reward distributor. This avoids having to lock every time we want to deal with it.
• Uses tracing::instrument to avoid repeating the same parameters when logging.
• Simplifies the logic around fetching the contract state (specifically the available budget and remaining budget). This was being fetched at different points in the code and this made it annoying to deal with. Now it's fetched at most once at the beginning of each loop. This also removes the Options around this info and keeps directly the u64 and U256 to avoid dealing with options since there was no need for them.
• Stops caching the round information since there is no need for this as we only deal with each key/round once.
• Adds a couple of missing metrics around contract state.

[jimouris]

What is this, block number? seconds? I'll derive it from the context later on but might be helpful to have a comment or rename to something more descriptive.

[mfontanini]

Will fix in next PR, need to fix something

[jimouris]

I see.

[jcabrero]

👍