chore: generalise session pool class for multiplexed session. (googleapis#2964)

* chore: generalise session pool class for multiplexed session.

* chore: add back previous code.

* chore: address comments.

Author: arpan14

google-cloud-spanner/src/main/java/com/google/cloud/spanner/DatabaseClientImpl.java

@@ -257,7 +257,9 @@ private <T> T runWithSessionRetry(Function<Session, T> callable) {
       try {
         return callable.apply(session);
       } catch (SessionNotFoundException e) {
-        session = pool.replaceSession(e, session);
+        session =
+            (PooledSessionFuture)
+                pool.getPooledSessionReplacementHandler().replaceSession(e, session);
       }
     }
   }