[storage] Only update last reconciled if index >

[patrick-ogrady]

Related: #187

As a result of using buffered channels in the reconciler, we need to protect against "out of date" updates to the last_reconciled information in storage.

Changes

• Only update last reconciled if index >
• Add block exists method to block_storage (enables block pruning before reconciliation)
• Change BlockExists to CanonicalBlock in reconciler

[heimdall-asguard]

If Heimdall is stuck, please click the following link: https://heimdall-public.cbhq.net/v1/pull_request/stuck?pr_number=194&repository=coinbase/rosetta-sdk-go

[coveralls]

Pull Request Test Coverage Report for Build 9185

• 21 of 25 (84.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.006%) to 78.324%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
storage/block_storage.go	14	18	77.78%

Totals
Change from base Build 9118:	0.006%
Covered Lines:	6710
Relevant Lines:	8567

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💛 - Coveralls