Fix container creation race condition

Fixes #249

This (thankfully) ended up being much simpler than I had originally thought. The transaction chunks for container creation were being sent asynchronously, which interestingly manifested differently in different browsers. This change makes each chunk wait until the last has confirmation from the server.

It is a little daunting to write a test for this, requiring a SQL query in the integration tests, which aren't set up for it. Being such a small change I decided against it.