pool: Add transaction helpers and support

[TheThing]

This adds some much needed support for doing transactions from a pool with a few helper utilities.
Notable new API methods:

pool.beginTransaction(function (err, description) { /* description.connection.query() */ })

Pulls a connection information out of the pool and gives it to the caller. This connection becomes busy for the duration until either of next two functions are called:

pool.commitTransaction(description, function(err) { /* err logging */ })

Calls IF (@@TRANCOUNT > 0) COMMIT TRANSACTION on the connection in the description and releases it back into the pool allowing it to be used by others.

pool.rollbackTransaction(description, function(err) { /* err logging */ })

Calls IF (@@TRANCOUNT > 0) ROLLBACK TRANSACTION on the connection in the description and releases it back into the pool allowing it to be used by others.

Both functions are also available in the promised pool:

let description = await pool.promises.beginTransaction()
try {
  await description.connection.promises.query(`
    SELECT * from my_table WITH (updlock, holdlock)
  `)
  // do some other stuff
  await pool.promises.commitTransaction(description)
} catch (err) {
  await pool.promises.rollbackTransaction(description)
  // log err
}

Lastly, a nice little promise-specific utlity has been added:

pool.promises.transaction(async function (description) {
  /* I'm inside a transaction */
  await description.connection.promises.query(`Do transaction query here`)
})

This calls the callback and wraps it inside of a transaction with auto commit on success and auto rollback on any thrown errors.

[TheThing]

This closes #353

[TimelordUK]

this looks great, will hopefully pull over weekend - thanks a lot.

[TheThing]

Thank you, I wanted to add unit tests for all this functionality but I couldn't get them to run. After npm install it complained it couldn't find const sql = require('msnodesqlv8') module in env/CommonTestFunctions and I didn't know how to fix that :(

Otherwise I would have added unit tests for all of these