Merge common impls of `submit_one_event` into `submit_event_chain`

[vmingchen]

What does this PR do?

Address https://github.com/DataDog/glommio/issues/548

Motivation

Refactor code to reduce code duplication.

Related issues

A list of issues either fixed, containing architectural discussions, otherwise relevant for this Pull Request.

Additional Notes

Pure refactoring; no behavior change.

Checklist

[] I have added unit tests to the code I am submitting [] My unit tests cover both failure and success scenarios [] If applicable, I have discussed my architecture

[bryandmc]

Hey @vmingchen -- In the future, can you either fill out the pull request template or at least delete it (if it's not relevant, though that's also debatable..)? It was surprising to come read this and realize it was just the template unchanged.

[vmingchen]

Hey @vmingchen -- In the future, can you either fill out the pull request template or at least delete it (if it's not relevant, though that's also debatable..)? It was surprising to come read this and realize it was just the template unchanged.

Sorry about that. Edited the PR description.

[glommer]

anything missing for merging this ?