Asynchronous `unlock`.

Upgrade to Cadence 0.0.38 and Strata 0.0.20. I needed to go back to
Strata and upgrade it's Cadence to 0.0.38 because the Strata 0.0.19
`gather` test helper function breaks on Cadence 0.0.38 behavior.
Basically, there was too much array wrapping in the `gather` function
returns so I updated Cadence to simpifly matters.

Closes #18.