Remove usage of the transitive dependency Optimal

[kamilkowalski]

Relates to spandex-project/spandex#124. Currently spandex_datadog makes use of Optimal even though it doesn't list it as a dependency - it's only available because spandex requires it, and spandex_datadog requires spandex. This removes the use of Optimal, as we think it is the way to go for Spandex to be fast.

[GregMefford]

This looks good to me. Since we're only validating the params in start_link, this change itself won't break any existing code (it would have to already be broken to make a difference), but now that we're not validating the opts, it's possible that someone could configure invalid opts and cause a crash somewhere else where it might be less obvious what the problem is.

I'm thinking that we can bundle this change with the re-work of the ApiServer (#32) together and release a new major/minor version once we figure out what all the changes will look like.

[kamilkowalski]

Awesome! That PR looks interesting - together with removing Optimal could boost performance significantly.

[GregMefford]

Thanks again for taking the time to create this PR and sorry it took so long to get it merged! 🎈 🚀