Ensure that transparent funds are protected when moving to a shielded-only librustzcash build.

[nuttycom]

Add an explicit check for unspent transparent outputs whenever running initialization with a version of the zcash_client_sqlite library not compiled with the transparent-inputs feature flag, and fail if any are present.

Originally posted by @daira in https://github.com/zcash/librustzcash/pull/600#discussion_r946243123