Find cache database using task parents

The endpoint RetrieveTaskFromAgent should give access to the task parents from the current task:

0. when use_cache is True
1. hit that endpoint during configure and store the parent IDs
2. merge step:
   • if there is only one parent task, copy that file into the /data/current folder
   • otherwise, merge immediately all the database files into a single one into the /data/current folder

merge code

def merge_databases(source_db, dest_db):
    """
    Low-level method to merge two mbtiles (sqlite databases)
    """
    assert os.path.exists(source_db)
    assert os.path.exists(dest_db)

    statements = [
        "PRAGMA journal_mode=PERSIST;",
        "PRAGMA page_size=80000;",
        "PRAGMA synchronous=OFF;",
        "ATTACH DATABASE '{}' AS source;".format(source_db),
        "REPLACE INTO map SELECT * FROM source.map;",
        "REPLACE INTO images SELECT * FROM source.images;",
    ]
    connection = sqlite3.connect(dest_db)
    cursor = connection.cursor()
    for statement in statements:
        cursor.execute(statement)
    connection.commit()
    connection.close()