zcash_client_backend: Add an "account birthday" parameter to the create_account API.

[str4d]

As part of this implementation, the init_blocks_table and init_accounts_table APIs should be removed in favor of initialization using the normal methods of the data access API, add_account and update_chain_tip. By performing these removals, we can then establish the invariant that the blocks table only contains entries for blocks that the wallet has scanned, which will be a useful property.

[str4d]

This issue will need to result in an additional error state indicating that the caller needs to first call update_chain_tip so that newly-created accounts can have birthdays assigned to them (as opposed to ScanRequired which indicates the caller needs to call suggest_scan_ranges).

[nuttycom]

Fixed by #907