Support System.Transactions.

[bgrainger]

Adds support for ambient transactions, and for explicitly enlisting with EnlistTransaction.

Unlike Connector/NET, this is implemented using MySQL's XA Transactions. This may be a compatibility problem for users migrating from Connector/NET, but better matches the purpose/intent of TransactionScope.

Currently only implemented for .NET Framework 4.5.1, since .NET Core won't support System.Transactions until netstandard-2.0: dotnet/corefx#10089

[caleblloyd]

Very nice addition to MySqlConnector!

[bgrainger]

I'm going to implement the AutoEnlist connection string setting before merging this. Because this implementation uses XA Transactions it's subject to these restrictions and clients can't use these statements during one. Supporting AutoEnlist will let clients opt into Connector/NET compatible behaviour as follows:

Old:

using (var ts = new TransactionScope())
using (var conn = new MySqlConnection(connStr))
{
    conn.Open();
    // ...
    ts.Complete();
}

Compatible:

using (var ts = new TransactionScope())
using (var conn = new MySqlConnection(connStr + ";AutoEnlist=False"))
{
    conn.Open();
    using (var mySqlTrans = conn.BeginTransaction())
    {
        // ...
        mySqlTrans.Commit();
        ts.Complete();
    }
}

[bgrainger]

Implemented in 0.18.0.