TxTracking should not modify the client's options object

[dkent600]

The TxTracking system should not modify the client's options object that is supplied to every Arc.js function that generates a transaction. Currently it inserts a property called txEventContext.

This change may require adding an optional argument to every Arc.js function that generates a transaction.

The only known alternative is to document that Arc.js adds this property, and perhaps rename the property to be more clear about where it came from.