Enforce a maximum size for the replacement mapping.

If the replacement mapping becomes too large after the execution of a
transaction, it is reset, and the parent mapping is reconstructed. This is a
performance optimization which aims to ensure that the access to parent nodes
does not become too complex.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/configuration/branches/immutableNodes@1568834 13f79535-47bb-0310-9956-ffa450edef68