x/superfluid: ensure events are emitted and tests added

[hieuvubk]

Closes: #2194

What is the purpose of the change

Brief Changelog

• Refactor and unit test superfluid module events
• Write test to make sure events emitted in right place
• Update readme

Testing and Verifying

This change added tests and can be verified as follows:

• Internal events unit tests
• msg server tests

Documentation and Release Note

• Does this pull request introduce a new feature or user-facing behavior changes? (yes / no)
• Is a relevant changelog entry added to the Unreleased section in CHANGELOG.md? (yes / no)
• How is the feature or change documented? (not applicable / specification (x/<module>/spec/) / Osmosis docs repo / not documented)

[p0mvn]

I would like to have one more reviewer give this a look before merging since the diff is large. Overall, the change LGTM. Also, I left a comment for follow-up work that would be good to decide on