Increase mongo performance

[JoranAngevaare]

Change the following lines:
Change this line https://github.com/AxFoundation/strax/blob/master/strax/storage/mongo.py#L112 to read 'chunk_i':{"$ge":0} for this we need to create a 'chunk_i'== - 1 when we create a metadata entry

See if we can update this op. https://github.com/AxFoundation/strax/blob/master/strax/storage/mongo.py#L281

Further we should also do the following:

• collection.delete_many(query) before inserting the new meta-data document. Only one copy of the metadata should be allowed
• add a provides_metadata field to query against. This solves using slow operations as in Temp fix query for md queries #374.