Support threaded receipts

[bnjbvr]

Right now, the SDK assumes in multiple places that we only support unthreaded receipts. We also try our best to be compatible with receipts threaded from the main thread.

When we'll want to support threaded receipts, we'll need to modify a bunch of code, so I'm opening this issue so as to be able to link to it from the code.

To make expectations clear: it's not a priority for us at the moment to support threads or threaded receipts, as far as I understand (but as usual, contributions would be welcome :)).