Basic API Rate Limiting

Implement a token bucket rate limiter for a Luxoft API endpoint. Each user has an independent bucket that refills over time and allows a request only when at least one token is available.

Write allow_request(limiters, user_id, timestamp, capacity, refill_rate):

1. limiters is a mutable dictionary storing per-user bucket state. It may initially be empty.
2. user_id is a hashable identifier.
3. timestamp is a non-negative number of seconds. Calls for the same user are provided in non-decreasing timestamp order.
4. capacity is the maximum number of tokens in each bucket.
5. refill_rate is the number of tokens added per second.
6. A new user's bucket starts full.
7. Each accepted request consumes exactly one token.
8. Return True when the request is accepted, otherwise return False.
9. Update the user's bucket state regardless of whether the request is accepted. Do not simulate time continuously. Refill lazily when a request arrives.

The function should use constant additional work per request, excluding dictionary resizing.