Read the virtualchain snapshots
    Returns the dict of {snapshots: {$block_height: $consensus_hash}} on success
    Returns None on error

def get_snapshots(working_dir, start_block=None, end_block=None):
    """
    Read the virtualchain snapshots
    Returns the dict of {snapshots: {$block_height: $consensus_hash}} on success
    Returns None on error
    """

    # make this usable even if we haven't explicitly configured virtualchain 
    impl = sys.modules[__name__]
    return BlockstackDB.get_consensus_hashes(impl, working_dir, start_block_height=start_block, end_block_height=end_block)