Index pattern/loading 2

[w33ble]

Replaces #1601
Fixes #1561
Fixes a bug introduced by aca2457.

The code in aca2457 assumes that SearchSources would have an indexPattern defined on them. This is the case with savedSearches but not for visualizations. Rather than use the indexPattern saving build into the SavedObject, they manage a separate indexPattern property, and link it to the SearchSource in the SavedObject#afterEsResp() callback.

This fixes the bug with the following updates:

• SavedVis no longer manages it's own indexPattern, it is managed by the underlying SavedObject
• searchSource.get('index') will always work synchronously
  • Previously, if a searchSource inherited it's index pattern from the global searchSource, it would return undefined.
• rootSearchSource does not update it's index pattern when the config is changed
  • it now happens in routeSetup, which is what allowed us to make the calls to DataSource#getParent() and DataSource#get() behave synchronously.