Tx/VP prefix iter should include keys from write log

[tzemanovic]

The prefix iterator for Tx and VP posterior state should also iterate over the storage keys from write log. Currently, they only iterate over the keys committed to the storage, but replace any overwritten values from write log.

┆Issue is synchronized with this Asana task by Unito

[tzemanovic]

not sure if this one is actually a "good first issue", as the host env implementation employs some less-common concepts

[tzemanovic]

this is blocking #740