Replace per-session pool trimming with pressure hysteresis

Co-Authored-By: brekotis <93345790+brekotis@users.noreply.github.com>

src/proxy/middle_relay/session.rs

@@ -823,7 +823,6 @@ where
 
     clear_relay_idle_candidate_in(shared.as_ref(), conn_id);
     me_pool.registry().unregister(conn_id).await;
-    buffer_pool.trim_to(buffer_pool.max_buffers().min(64));
     let pool_snapshot = buffer_pool.stats();
     stats.set_buffer_pool_gauges(
         pool_snapshot.pooled,