Drop test data

* Added test data for the droplets tables i.e. the main
drops table ("droplets") and the metadata tables -
droplets_tags, droplets_places, droplets_links, droplets_media.
The data for each of the above, are in their own script files

* Re-organized the execution order of the initialization scripts
for the test database
Delete IdentityDTO.java

* The DTO classes for identities, tags, links, media
and places are now nested classes in DropDTO.java
Endpoints to fetch and delete drops from bucket

* Implemented the service methods and accompanying
DAO contracts for fetching and deleting drops from a
bucket via the API. For the optional parameters, these
are stored in a Map<String, Object> and validation is performed
by BucketService. At the DAO level, the predicates are dynamically
built out based on the specified query parameters.

* TODO Add support for date_from and date_to parameters.