Send heartbeats from queue as batches

[johan-bjareholt]

Will improve performance greatly when the queue is long

[TimeToBuildBob]

For context, the current queue implementation (`RequestQueue`) sends heartbeats one at a time from the persist queue. When the server is unavailable, heartbeats accumulate in the SQLite-backed queue and are drained sequentially after reconnection.

Batching would help in two scenarios:

1. Large queue drain: After a long server outage, thousands of queued heartbeats need to be sent. Batching reduces HTTP overhead.
2. High-frequency watchers: Watchers with short poll intervals generate many heartbeats — batching reduces server load.

The implementation would be in `RequestQueue._dispatch_request()` — instead of calling `self.client._post(endpoint, data)` for each request individually, collect N requests (up to some batch size) targeting the same bucket and send them together via the bulk insert endpoint (`POST /api/0/buckets/{id}/events`).

Note: heartbeats specifically need the heartbeat endpoint (which does server-side merging), so batching is trickier there — you'd need to either batch as regular inserts (losing server-side merge) or add a bulk heartbeat endpoint to the server.