feat: Migrate ConnectionManager to TypeScript

[ephys]

Pull Request Checklist

• Have you added new tests to prevent regressions?
• Does yarn test or yarn test-DIALECT pass with this change (including linting)?
• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Did you update the typescript typings accordingly (if applicable)?
• Does the description below contain a link to an existing issue (Closes #[issue]) or a description of the issue you are solving?
• Did you follow the commit message conventions explained in CONTRIBUTING.md?

Description Of Change

This PR migrates the AbstractConnectionManager to TypeScript. As usual it isn't just a type addition but a small refactor to work better with TypeScript.

Notable Change 1: Pool Normalization

A first notable change is the unification of how the connection pool works with and without replication.
Before this PR, if read replication was disabled, connectionManager.pool was an instance of sequelize-pool. If read replication was enabled, connectionManager.pool was an object that mimicked sequelize-pool and used two pools internally (one for read, one for write).

After this PR, connectionManager.pool is always an instance of ReplicationPool (new class) which dispatches to its internal read and write pools. If read replication is disabled, the read pool is disabled and the write pool is used for reads too.

This change makes the type much easier to work with.

Notable Change 2: Sequelize option priority

Which parameter takes priority in the Sequelize constructor is inconsistent:

// The following resulted in 'sequelize.config.username' being equal to 'user1'

new Sequelize('db1', 'user1', 'pwd1', {
  username: 'user2',
});

// The following resulted in 'sequelize.config.username' being equal to 'user2'

new Sequelize('mysql://user1:pwd1@localhost:1234/dbname', {
  username: 'user2',
});

After this PR, both versions prioritize the option bag over the string parameters. Both result in user2 being used.

Notable Change 3: Fixing another MySQL schema bug

Moved to #14657. The code is still in this PR but that other PR should be merged first.

---

Closes #13660

Squash Commit Message Body

If this is merged by someone else than me, here is the message body to use for the squash commit (don't forget the empty line at the start of the body, per conventional commits conventions):

BREAKING CHANGE: The signature of `connectionManager.pool` when read replication is disabled is now the same as when read replication is enabled. If read replication is disabled, you can access the actual pool through `connectionManager.pool.write`. If read replication is enabled, you can use both `connectionManager.pool.read` and `connectionManager.pool.write`.

[WikiRik]

Good work on this already! Let's indeed move the MySQL schema bug to another PR

[ephys]

Moved to #14657

[WikiRik]

@ephys I'll leave the merging up to you

[github-actions]

🎉 This PR is included in version 7.0.0-alpha.15 🎉

The release is available on:

• npm package (@release-v7 dist-tag)
• GitHub release

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