Clean up shared DB statements code between Mysql2 and Trilogy

[adrianna-chang-shopify]

Motivation / Background

There is a lot of duplicated code between ActiveRecord::ConnectionAdapters::MySQL::DatabaseStatements (used by Mysql2Adapter) and ActiveRecord::ConnectionAdapters::Trilogy::DatabaseStatements. This PR introduces a DatabaseStatements module at the AbstractMysqlAdapter level, and moves this shared code into it.

Detail

This PR makes two changes:

1. Renames the existing MySQL::DatabaseStatements module to Mysql2::DatabaseStatements, since it's Mysql2-specific rather than MySQL specific. I did this because I'd like to use MySQL::DatabaseStatements to hold shared database statement logic for all MySQL-compatible adapters.
2. Moves shared logic out of the Trilogy + Mysql2 database statement modules and into a new ActiveRecord::ConnectionAdapters::MySQL::DatabaseStatements module. This involves removing some code that was in the TrilogyAdapter that ought to have lived in the Trilogy::DatabaseStatements module (e.g. multi-statement support).

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[adrianna-chang-shopify]

There are test failures around explain options that I need to investigate 🤔