use badgerdb on mobile devices

[dreacot]

closes #694

• use badger db for mobile devices because Bolt db crashes when wallet database size is getting close to 250mb. At that point, it's unable to open wallet because the allocated app memory cannot handle that. The crash usually starts from when the wallet is fetching headers and the wallet db rises to the size which the app's allocated memory cannot handle. With badger, a database can be split into multiple files which makes access faster and uses less memory than bolt database