FIX Repeat calls to mssql.connect() will only resolve when the pool is connected

[dhensby]

This is an improvement on #804

Since the previous PR repeated calls to mssql.connect() would resolve instantly even if the pool wasn't yet in a connected state. eg:

const pool = mssql.connect().then(() => console.log('first'));
const pool2 = mssql.connect().then(() => console.log('second'));

// output:
// second
// first

This has lead to many issues where developers are not taking into account that they shouldn't be relying on being able to call connect() repeatedly.

This will improve developer experience as they will need to think less about managing the state of their global connection

todo:

• tests

[dhensby]

NB: This code can be greatly simplified once #941 is completed