zcash_keys: Migrate transparent gap limit address generation rules from `zcash_client_sqlite` to `zcash_keys`

[nuttycom]

Correct gap limit handling is tricky, and is good to standardize for the wallet ecosystem. The gap limit handling logic in zcash_client_sqlite can be abstracted to take advantage of a capability-oriented approach to the wallet database interaction, and then that logic can be extracted into the zcash_keys crate so that others can make use of it. This will also be advantageous for the maintenance of the zcash_client_memory crate, which has not yet implemented general gap limit handling logic.