Define default Adapter#name

[bkeepers]

I noticed while working on #760 that every adapter defines #name and it's just a variable set in initialize. This PR added a default Adapter#name method, which is just the snake_case version of the ClassName.

In #760 (comment), @jnunemaker said:

I don't usually allow changing the adapter name. But seeing it makes me wonder if we should allow this for people as I could see scenarios where someone wants to override for instrumentation purposes.

Currently, the ActiveRecord and Sequel adapters are the only ones that allow setting the name. Those will still work with this change. Should we allow others?

[jnunemaker]

I think this should work. As long as the names are the same for anyone instrumenting we should be fine. If the magic changed any names, we'd want to let people know in changeling as metric names would change and they'd have to update graphs.

[bkeepers]

👍 All names should be the same.