Refactor BackoffSettings

[blowmage]

• Make BackoffSettings a class with optional named arguments for the constructor, instead of a Struct with positional arguments without a default value.
• This will allow users to modify only the values they care about when creating a new BackoffSettings object.
• Change BackoffSettings to use seconds instead of milliseconds.
• Remove MILLIS_PER_SECOND as it is no longer needed.
• Remove google/gax/constants.rb file.

[codecov[bot]]

Codecov Report

Merging #168 into master will decrease coverage by 79.29%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #168      +/-   ##
==========================================
- Coverage   95.77%   16.47%   -79.3%     
==========================================
  Files          14       11       -3     
  Lines        1066      977      -89     
==========================================
- Hits         1021      161     -860     
- Misses         45      816     +771

Impacted Files	Coverage Δ
spec/google/gax/operation_spec.rb	0.74% <0%> (-99.26%)	:arrow_down:
lib/google/gax/operation.rb	2.85% <0%> (-97.15%)	:arrow_down:
spec/google/gax/util_spec.rb	0.95% <0%> (-99.05%)	:arrow_down:
spec/google/gax/api_callable_spec.rb	1.9% <0%> (-95.24%)	:arrow_down:
spec/google/gax/grpc_spec.rb	17.72% <0%> (-82.28%)	:arrow_down:
lib/google/gax.rb	25% <0%> (-75%)	:arrow_down:
spec/google/gax/error_spec.rb	26.71% <0%> (-73.29%)	:arrow_down:
lib/google/gax/grpc.rb	35.29% <0%> (-58.83%)	:arrow_down:
... and 3 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 624cc20...028cfbf. Read the comment docs.

[codecov[bot]]

Codecov Report

Merging #168 into master will decrease coverage by <.01%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #168      +/-   ##
==========================================
- Coverage   95.77%   95.77%   -0.01%     
==========================================
  Files          14       13       -1     
  Lines        1066     1065       -1     
==========================================
- Hits         1021     1020       -1     
  Misses         45       45

Impacted Files	Coverage Δ
lib/google/gax.rb	100% <ø> (ø)	:arrow_up:
spec/google/gax/operation_spec.rb	100% <100%> (ø)	:arrow_up:
lib/google/gax/operation.rb	100% <100%> (ø)	:arrow_up:
lib/google/gax/settings.rb	88.88% <100%> (+2.4%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 624cc20...028cfbf. Read the comment docs.

[blowmage]

BackoffSettings isn't used in retry AFAIK. I think it is only used in Operation#wait_until_done!.

[blowmage]

Oh, you are right. They were included in RetryOptions before it was removed. I can sit on this change until we implement the updated error retry.

[blowmage]

Adding do not merge until #166 is approved and merged. This PR will need to be rebased to fix conflicts.

[blowmage]

This is satisfied by #178.