Provide a new wrapper fdb_get API that doesn't require an application to create a fdb_doc instance

The current fdb_get API requires an application to create fdb_doc instance and pass it to fdb_get API to get the metadata and doc body for a given key. In addition to this, we need to provide a new wrapper fdb_get API that creates fdb_doc instance and returns it to the application.

The definition of this new API should be as follows:

fdb_status fdb_get(fdb_handle *handle, void *key, size_t keylen, fdb_doc **pdoc);