Timeout when downloading initial data

[cmelchior]

When using synchronized Realms you can specify

user.createConfiguration(url)
  .waitForInitialRemoteData()
  .build();

This means the Realm will not be opened before any relevant server data has been fully downloaded. In cases of poor/no network this can, however, take quite a lot of time. In those cases, a user might want to abort and do something else.

The download will react to either calling Thread.interrupt() or RealmAsyncTask.cancel(), but it is rather manual and annoying to have to implement timing yourself.

Instead, we should make it possible to define a timeout parameter that will abort automatically, like:

user.createConfiguration(url)
  .waitForInitialRemoteData(5, Timeunit.SECONDS)
  .build();

This is also related to #6073