Use binary keys to protect data

[linuxwolf]

In order to support "FxA as Master Password" (mozilla-lockwise/lockbox-extension#209), the datastore needs to support encryption using a binary key.

• When initialize() is called, allow the API user to pass in a JWK or Uint8Array that represents a symmetric key in order to prepare the databases.
  • Allow the API user to call initialize() without a key, and use a consistent "null" key value to support the low-touch "quickstart"
• When unlock() is called, allow the API user to pass in a JWK or Uint8Array that represents the symmetric key used when initializing.
  • Allow the API user to call unlock() without a key, and use the default "null" key value