optimize timer allocations with stream pools

Stream pool implementation creates a timer in each iteration.
https://github.com/cockroachdb/cockroach/blob/6806299943a595425030f0ad3c54ccc680c89842/pkg/rpc/stream_pool.go#L127-L133

Each stream in the pool waits for a request to be sent before it timeout after 10s or the context is cancelled. If a request arrives within idle timeout, we create a timer unnecessarily. As of Go 1.23, the garbage collector will reclaim any unreferenced, unexpired timers, so we no longer have to worry about leaking timers. But we still room to avoid these allocations potentially by pooling them.

Jira issue: CRDB-50568