Migrate stateful operators to use pipeline state mechanism

[anupdhml]

Describe the problem you are trying to solve

Operators that maintain state can be migrated to use the pipeline state mechanism (eg: for the batch or backpressure operator) -- this would be a necessity when we want to support pipeline migration to a different tremor node, when we have clustering for tremor.

Describe the solution you'd like

Use the pipeline state mechanism for these operators:

• generic::batch
• generic::backpressure

Come up with a solution for these operators, that have internal state not easily expressible as Value (our primitive for the operator state). One solution is to serialize the stateful data structures into (and deserialize out) of the pipeline state, when we do need to store them somewhere central (eg: to enable pipeline migrations as part of clustering effort).

• grouper::buckert (maintains LRU cache)

Notes

Pulled from https://rfcs.tremor.rs/0002-pipeline-state-mechanism/#future-possibilities to track as an issue.

Also see https://rfcs.tremor.rs/0002-pipeline-state-mechanism/#unresolved-questions

[anupdhml]

For reference, generic::counter operator is already using the state mechanism:
https://github.com/wayfair-tremor/tremor-runtime/blob/v0.7.3/tremor-pipeline/src/op/generic/counter.rs

Implementation PR it was part of: #59