Store indexes on the local file system (using MapDB)

Implement the new IndexProvider SPI to store relevant configured indexes locally on the file system using MapDB, and thus no additional dependency is required. This will allow users to define specific indexes and say which (if any) should be stored on the local file system.

The provider should support indexes for normal property types: STRING, LONG, DOUBLE, DATE, DECIMAL, URL, PATH, REFERENCE, WEAKREFERENCE and SIMPLEREFERENCE. There is no need for BINARY properties or full-text search.