Construction API Coordinator

[patrick-ogrady]

Related Issue: coinbase/mesh-cli#112

This PR includes an implementation of the coordinator (the core processor for the constructor). To better test this coordinator, this PR also includes a refactor of the constructor into two separate packages executor and coordinator.

Changes

• Implement coordinator which orchestrates the execution of jobs, the creation of jobs, and the running of reserved workflows
• Add tests for coordinator
• AllBroadcasts should be a function on JobStorage instead of BroadcastStorage
• Test 2-step job

[coveralls]

Pull Request Test Coverage Report for Build 4874

• 221 of 332 (66.57%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.6%) to 76.726%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
constructor/executor/worker.go	20	31	64.52%
constructor/coordinator/coordinator.go	201	301	66.78%

Totals
Change from base Build 4763:	-0.6%
Covered Lines:	4523
Relevant Lines:	5895

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💛 - Coveralls