mvcc: add release note for index compaction changes

CHANGELOG-3.4.md

@@ -34,6 +34,7 @@ See [code changes](https://github.com/coreos/etcd/compare/v3.3.0...v3.4.0) and [
   - e.g. a node is removed from cluster, or [`raftpb.MsgProp` arrives at current leader while there is an ongoing leadership transfer](https://github.com/coreos/etcd/issues/8975).
 - Add [`snapshot`](https://github.com/coreos/etcd/pull/9118) package for easier snapshot workflow (see [`godoc.org/github.com/etcd/snapshot`](https://godoc.org/github.com/coreos/etcd/snapshot) for more).
 - Improve [functional tester](https://github.com/coreos/etcd/tree/master/functional) coverage: [proxy layer to run network fault tests in CI](https://github.com/coreos/etcd/pull/9081), [TLS is enabled both for server and client](https://github.com/coreos/etcd/pull/9534), [liveness mode](https://github.com/coreos/etcd/issues/9230), [shuffle test sequence](https://github.com/coreos/etcd/issues/9381), [membership reconfiguration failure cases](https://github.com/coreos/etcd/pull/9564), [disastrous quorum loss and snapshot recover from a seed member](https://github.com/coreos/etcd/pull/9565), [embedded etcd](https://github.com/coreos/etcd/pull/9572).
+- Improve [index compaction blocking](https://github.com/coreos/etcd/pull/9511) by using a copy on write clone to avoid holding the lock for the traversal of the entire index.
 
 ### Breaking Changes