Creating a staged insert is using an exponential delay

The spec requires a 3ms delay but an exponential delay is being used instead.  For users wanting low transaction latencies this is going to impact it too much on this path.

On trying to change it, the change worked fine locally, but caused a crash on CI on particular FIT tests, and was reverted under https://review.couchbase.org/c/couchbase-transactions-cxx/+/169550.

In real-world use we should never be able to tight-loop enough to cause stack recursion issues, since it'll be KV timeouts causing this issue.  It's only because FIT is artificially injecting the error.  Perhaps we need to make some adjustments on the FIT side - or avoid the recursion in the implementation.