feat: more accurate gas-oracle gas metering

[tynes]

Description
Compute the gas used in an epoch based on the
actual amount of gas used instead of assuming
that the full block's worth of gas was used in
each block.

Convert the average block gas limit to a uint64 in the
codebase instead of it being used as a float64.

Additional context
Minimal changes were done in a way where the existing tests
could ensure there is no regression. The gas-oracle is not
part of the integration-tests so thorough review and observing
its behavior on kovan is necessary before rolling this out to mainnet.
#1736 is a prerequisite
for getting the gas-oracle into the itests

Metadata

• Fixes ENG-1681

[changeset-bot]

🦋 Changeset detected

Latest commit: ab2378e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@eth-optimism/gas-oracle	Patch

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[codecov-commenter]

Codecov Report

Merging #1813 (ab2378e) into develop (84513e7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #1813   +/-   ##
========================================
  Coverage    71.99%   71.99%           
========================================
  Files           70       70           
  Lines         2321     2321           
  Branches       346      346           
========================================
  Hits          1671     1671           
  Misses         650      650           

Flag	Coverage Δ
batch-submitter	62.07% <ø> (ø)
contracts	87.96% <ø> (ø)
core-utils	57.50% <ø> (ø)
data-transport-layer	38.64% <ø> (ø)
message-relayer	70.86% <ø> (ø)

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[mslipper]

LGTM from my POV - may want get Karl to take a look as well for sanity.

[karlfloersch]

LGTM! Left a tiny bit of feedback but overall all of the logic looks right.