quic tpu: create fewer timeout futures (#3151)

* 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.

* quic tpu: lower stream timeout from 10s to 2s

Don't wait 10s for one piece of a transaction to come before timing out.
Lower the timeout to 2s, after which we assume a stream is dead. 2s is
enough round trips to account for non catastrophic packet loss. If
packet loss is causing >2s stream latency, the connection is hosed and
the best thing we can do is save server resources for peers with better
connectivity.

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]