Slow delivery might block message buffering

[samber]

In client.go, flushing the buffer to the log sink can be very slow, depending on network congestion and latency.

During that time, the unbuffered channel c.entries will block the main process from emitting log.

I think sendBatch should be called in a goroutine, except for final flush.

Linking the issue to samber/slog-loki#18 and samber/slog-loki#19

WDYT ?