Allow to disable mounting path fragments in the MultiplexingNodeStore

Let's assume we have a working instance with multiplexing in place, the /libs is mounted in the read-only mode. Now, when an index is created or modified, the whole repository (including /libs) will be reindexed. Then, the IndexUpdate will try to store part of the generated data to /oak:index/lucene/:oak:mount-libs-index-data. The MultiplexingNodeStore won't allow this (as the libs store is a read-only one) - so the whole reindexing operation will fail and be started again.

A solution proposal: let's allow to disable mounting the path fragments. Indexing data for the libs mount will be stored on the main repository, but under a separate path.