quic tpu: create fewer timeout futures

Use a cancellation token instead of polling an Arc<bool> to exit
connection tasks. Before this change we used to create a timeout future
for each tx, and immediately cancel it since virtually all connections
always have incoming tx pending. Cancelling timeout futures is expensive
as it goes inside the tokio driver and takes a mutex. On high load (1M
tps), cancelling timeout futures takes 8% (!) of run time.

With this change we create a cancellation token _once_ when a connection
is established and that's it - no overhead after that.

streamer/Cargo.toml

@@ -38,6 +38,7 @@ solana-sdk = { workspace = true }
 solana-transaction-metrics-tracker = { workspace = true }
 thiserror = { workspace = true }
 tokio = { workspace = true, features = ["full"] }
+tokio-util = { workspace = true }
 x509-parser = { workspace = true }
 
 [dev-dependencies]