SQL DB based topology monitor

This is similar to KNOX-1012 but the providers/descriptors are stored in the database instead of zookeeper.

• If a provider/descriptor is deleted from the db but exists on the file system the monitor is going to delete it from the file system
• If a provider/descriptor exists in the DB but missing from the file system the monitor is going to download it from the DB to the FS.
• If a provider/descriptor in the DB has a different content than the corresponding local file, then the monitor is going to update the local file with the content from he DB. However to avoid too frequent and unnecessary IO operations we only change the local files if the remote change happened after the last synchronization time (if it happened in the past, we assume the changes were already synced to the disk). So one can change the local files on the disk manually without worrying about losing their local changes, until someone updates the DB.