Wallet: Add open/close and connect/disconnect events.

[nodech]

WalletDB open does not guarantee that the connection will open, if it's in standalone mode it may fail. So having the connect method inside open w/o any guarantees leaves any logic depending on it break. So it's much better to totally remove the concept of connection from open.

This changes several things:

• on open state cache needs to be recovered first (it was done on connect). If we don't have cache, then we initialize after on connect in the syncNode.
• migration can no longer start a rescan (and it may have failed previously on standalone) - it will instead rollback.
• Added new flag wallet-migration-no-rescan - that tells to ignore rescan request from the migration.

Restructure the life-cycle methods for the walletdb.

• open - no longer calls connect - now emits open event.
• connect - needs to be called separately - now emits connect event.
• disconnect - now emits disconnect event
• close - will try to close client if it's open - now emits close event.

[coveralls]

coverage: 68.555% (-0.1%) from 68.674% when pulling 1f2251e on nodech:wallet-lifecycles into 7bd2078 on handshake-org:master.

[rithvikvibhu]

Tested Bob with wallet in plugin and remote modes. Didn't have to change anything since wdb.connect are called by hsd in both opens