Buffers events for an ExSeq handler and sends them to Seq in batches.

A batch is sent once :batch_size events are buffered, and everything that's buffered is sent every :flush_interval seconds. Requests are made from a task, one at a time, so the Flusher keeps accepting events while waiting on Seq.

If Seq can't be reached or responds with an error, the batch stays in the buffer and is retried on the next flush. The buffer holds at most :max_buffer_size events, dropping the oldest beyond that. Batches that Seq rejects as invalid (HTTP 400 or 413) are dropped.

Everything buffered is also sent on Logger.flush/0 and on shutdown.

Flushers are started by ExSeq when a handler is added; there's no need to start one yourself.