Multiplexing store support in Property indexes

In Oak one can define an index anywhere in the repository under a special node name oak:index. For e.g. if you want a property index for sling:resourceType at root level then you can create the index at /oak:index/resourceType and this index would store the index content at /oak:index/resourceType/:index.

1. Writing - At time of commit the IndexEditor would need to decide where the indexed content for a given path should be stored. To start with it can make use of PathToStoreMapper to decide which node to use the indexed content. For e.g. for /libs the indexed content is stored under :index-pr and for /content :index-sr is used. This is simpler for PropertyIndex where IndexStoreStrategy can be passed the right node
2. Reading - At time of reading the QueryIndex implementation would need to provide a union cursor which can perform lookup from all such store directories for a given index.

Open Items

1. Supporting unique indexes
2. Introducing new oak:index - Node under oak:index node are special. Depending on parent path it might be possible that they would have to be created in both repositories. For e.g. /oak:index would have to be present in both PR and SR. While /content/foo/oak:index can live only in SR.