Possible resource exhaustion in CreateTransaction from wallet

For a request to `POST /api/v1/wallet/transaction`, there has been two isolated, reported problems:

* Extremely slow (minutes) [replicable]
* OOM error, crash [not replicable, unknown if attempted proper replication]

WIthout the user's wallet's addresses and request parameters used, it is harder to debug.

Memory and cpuprofiling the request could reveal some hot point, but there could also be some bad edge case that causes problems; a thorough inspection of the code may reveal the problem.

Start at the POST /api/v1/wallet/transaction entry point and inspect CreateTransaction from the wallet. Reproduce the reported slow request with the wallet addresses and request parameters, then collect memory and CPU profiles. Done means the resource-intensive path is identified and the slow or OOM behavior is resolved or clearly narrowed to a specific edge case.

Written by the indexing model from the issue text.