Replace at-tip checks with utils functions from the latest sdk

[septerr]

Fixes # .

Motivation

Rosetta CLI checks to see if it is at tip at a few different places. These checks are not consistent.

For instance, before constructing transactions, the CLI checks if it is a tip based on (1) if the stored current block is within tip delay or (2) block returned by network/status is at tip (it is within tip delay or has sync status of true) and it is same as the stored current block.

Before broadcasting transactions, the CLI checks if it is at tip based only on if the stored current block is within tip delay.

These inconsistent checks were leading to the CLI getting stuck during transaction broadcast for chains that use sync status rather than tip delays.

Solution

Use utility functions from the sdk's utils package to do at-tip checks consistently.

Open questions

[patrick-ogrady]

You'll also want to modify this: https://github.com/coinbase/rosetta-cli/blob/23e6daf53528a2695b05e6a7f584d4f5e31c1023/cmd/root.go#L272

[coveralls]

Pull Request Test Coverage Report for Build 6718

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 58.737%

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Totals
Change from base Build 6692:	0.0%
Covered Lines:	521
Relevant Lines:	887

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💛 - Coveralls