In the event that the pool to which change should be sent cannot
automatically be determined based upon the inputs and outputs of a
transaction, it is up to the caller to specify where change should
be sent.
This change makes it easier for third parties to make use of the Unified
key infrastructure without incurring a dependency upon the rest of the
`zcash_client_backend` interfaces.
Jack Grigg
Kris Nuttycombe
Andrew Arnott