MongoSearcher - preserve indexes on truncate

[codespill]

Description

When truncating MongoDB, collections are dropped, therefore all indexes are lost.
This pull request propose using remove() method instead of drop(), which will remove all documents whilst preserving indexes.

Additional info

Documentation on remove() method:

To delete all documents in a collection, pass an empty document ({}).

Source: https://docs.mongodb.com/manual/reference/method/db.collection.remove/

[glensc]

Squashed fixup commits, added helper to remove duplication in tests. LGTM