Make Index{Rule|Descriptor} store multiple or none property key ids

o Introduce a FAILED IndexProxy for each schema rule
o Make label scan rules not show up in core API
o @Ignore IndexRecoveryIT for now
Made IndexDescriptor part of IndexConfiguration
Integrate processing of label updates into IndexingService

o Add IndexUpdater.process(UpdateMode, NodeLabelUpdate)
Introduce IndexStatement to tie scope of IndexReaders to containing statement
Enable SchemaIndexProviders to register themselves as label indexers

o Rename SchemaIndexProvider.Descriptor to top-level IndexProviderDescriptor
o Changes DefaultSchemaIndexProviderMap to be a real map and replaced uses of getDefault*
   with IndexProviderDescriptor constants
o New naming scheme for in-memory package: {Label|Property}IndexProvider