[SDK] Move to newer PostMessage pattern

As a developer, I would like a more robust comms layer for the SDK so that it can be resilient to external messages that may be sent to the iframe.

## Additional Context
When we built the EXP plugin, we came up with a must more resilient pattern for post message which uses a setInterval to confirm that the receiving end has received everything they need before closing the port. We need to move the SDK to this more resilient post message pattern.

Start by locating the SDK's iframe postMessage communication layer and then compare it with the more resilient pattern used by the EXP plugin. Confirm which message paths and receiving-end acknowledgements need to change, and verify that messages are retained until the receiver has everything required before the port closes.

Written by the indexing model from the issue text.