Ensure that all operations emit correct amounts

[bisgardo]

Purpose

Ensure that all operations emit correct amounts such that the check:data check of the Rosetta CLI tool passes.

Changes

Many operation types had amount in their metadata instead of the operation itself. And for some odd reason, they had the fee as their amount which cancelled out the fee operation...

The following changes were included as part of this:

• Add support for querying contract balance.
• Cleanup: Only emit "fee" operation if it's amount is non-zero.

The changes have been picked from #24 to have a working version for P3, but also because the changes are mostly irrelevant for that PR.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.

[bisgardo]

A full rosetta-cli check:data check against a mainnet node is currently running with no errors as of block 250000. With an estimated 1 week runtime for checking the rest of the chain, I think that's enough confidence to merge this for now.