Add a way to change pool's default database for new connections

For pool connections it is common to call changeUser after getConnection to reset the state of the connection. It can also be used to switch databases by passing {database: "newDatabaseName"} to changeUser. This works fine but appears to change the pool's default database for new connections as well (see also https://github.com/sidorares/node-mysql2/issues/477). This is not that problematic except for the fact that the pool now becomes unusable after calling changeUser({database: }) with a database that does not exist anymore: connecting to the pool fails with ER_BAD_DB. This happens in my use case as the database name can be entered by users (and they may be wrong).

So, in summary:

pool = createPool({database: "exists"});
con = pool.getConnection(); // works, database=exists
con.changeUser({database: "does_not_exist"}); // fails but otherwise fine
con.release();

con = pool.getConnection(); // does not work! pool.config.connectionConfig.database still set to "does_not_exist"

My workaround is to wrap getConnection as follows, which is a bit of a dirty hack as it accesses private fields:

async getConnection(opts: {user: string, password: string, database: string}): Promise<PoolConnection> {
		(this.pool as any).pool.config.connectionConfig.database = "information_schema"; // always exists
		const con = await this.pool.getConnection();
		await con.changeUser({
			user: opts.user,
			password: opts.password,
			database: opts.database
		});
		return con;
	}

My preferred solution would be to have getConnection (or a new method, say createFreshConnection) optionally accept the parameters as changeUser, that when passed are either used for creating a new connection or lead to a call to changeUser right after creating the new connection.

Start by tracing the pool getConnection entry point and the connection changeUser behavior described in the example. Check how a failed database change affects pool.config.connectionConfig.database. Done means a failed changeUser does not make later pool connections unusable, with the requested connection parameters handled through a supported API rather than private fields.