Integrate walletdb and updated waddrmgr.

This commit converts the wallet to use the new walletdb package as well as
the updated version of the secure hierarchical deterministic wallet
address manager which uses the new namespace feature provided by waleltdb.
Address some review items.

- Distinguish between missing address and database error
- Wrap help text for wallet generation seed
- Don't rescan if there are no addresses
- Update --create message for legacy wallet import
Turn echo off when reading passwords