Refactor txToPairs into smaller functions

Also adds tests for those functions, and improve fee estimation.
Make scrypt parameters overridable/optional
Move keystore to legacy directory.
Switch to new waddrmgr package

This commit converts the wallet to use the new secure hierarchical
deterministic wallet address manager package.

The following is an overview of modified functionality:

- The wallet must now be created before starting the executable
- A new flag --create has been added to create the new wallet using wizard
  style question and answer prompts
- Starting the process without an existing wallet will instruct now
  display a message to run it with --create
- Providing the --create flag with an existing wallet will simply show an
  error and return
Add conversion from legacy txstore if it exists.
Update --create message for legacy wallet import
Turn echo off when reading passwords
minor fixes
distinguish between missing address and database error
wrap help text for wallet generation seed
don't rescan if there are no addresses