Add find_first(offset) method

[Siapran]

Motivating example:

void put_chunk(Packet const& chunk)
{
    std::memcpy(
        message.buffer + chunk.position * CHUNK_SIZE,
        chunk.data, CHUNK_SIZE);
    missing.reset(chunk.position);
}

bool complete()
{
    received =
        (received == 0 ? missing.find_first()
                       : missing.find_next(received - 1));
    return received == boost::dynamic_bitset<>::npos;
}

with find_first(offset), complete() becomes:

bool complete()
{
    received = missing.find_first(received);
    return received == boost::dynamic_bitset<>::npos;
}