MULE-8179: NullPointerException thrown when using persistent object store under high load

_ TreeBidiMap used to store file/key mapping is not thread safe and must be synchronized. All the operations must synchronize the access and maintain the ock until the files are properly write/read to avoid race conditions (this can affect performance)

Conflicts:
	core/src/main/java/org/mule/util/store/PersistentObjectStorePartition.java