Mining Rule Engine Scaffolding and Sync Rate Rule implementation

[coderofstuff]

Splitting the PR at #634 into multiple parts, this is part 1/3 with

• Mining Rule Engine scaffolding
• Sync Rate Rule implementation

Mining Rule

• A new construct that tracks some data or metrics and influences mining in some form if conditions are met
• It has these parts:
  • Rule: the mining-related strategy this rule will apply when triggered
  • Metric: the data that this rule cares about and attempts to influence
  • Trigger: the condition(s) upon which this rule will begin to apply
  • Recovery: the condition(s) upon which this rule will be lifted

Sync Rate Rule

Goal:
Allow mining even when the sink timestamp is outdated if the sync rate falls below a threshold

Rule:

Allow mining even if the sink timestamp is outdated. Accept blocks in Submit Block RPC.

Metric:

Field	Tracking
body_count	Already tracked in processing counters
header_count	Already tracked in processing counters
received_blocks	Not tracked, but calculated as max(delta.body_count, delta.header_count)
expected_blocks	Not tracked, but calculated based on interval_in_millis / target_time_per_block

Definitions:

• Sync Rate = received_blocks / expected_blocks
• sync rate threshold = 0.9

Trigger:

• finality is "recent" AND
• sync rate < THRESHOLD

Recovery:

• sync rate >= THRESHOLD

[michaelsutton]

LGTM