Update TransactionManager to support magic parameter

[devhawk]

Summary or problem description
neo-project/neo#1902 added overload of IVerifiable.Sign that takes an explicit magic value. Transaction Manager needs to be updated to take an optional magic value parameter to its constructor (defaulting to Protocol.Default.Magic if not provided) that is in turn passed to Tx.Sign in TransactionManager.Sign[Async]

Should probably wait until after #335 is merged before doing this

Where in the software does this update applies to?
RPC

[superboyiii]

Summary or problem description
neo-project/neo#1902 added overload of IVerifiable.Sign that takes an explicit magic value. Transaction Manager needs to be updated to take an optional magic value parameter to its constructor (defaulting to Protocol.Default.Magic if not provided) that is in turn passed to Tx.Sign in TransactionManager.Sign[Async]

Should probably wait until after #335 is merged before doing this

Where in the software does this update applies to?
RPC

#335 is merged. Now we can do this.

[devhawk]

This was fixed as part of #335: #335 (comment)