refactor(client): remove redundant timestamp parameter from poll_timeouts

Use internal monotonic time by default and introduce poll_timeouts_at for callers requiring a specific sampled Instant. This simplifies the API for standard usage while maintaining flexibility for external event loops.

crates/irtt-client/tests/protocol_options.rs

@@ -1,6 +1,5 @@
 mod support;
 
-use irtt_client::ClientTimestamp;
 use std::time::Duration;
 
 use irtt_client::{