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)
Merge pull request #1327 from mulesoft/MULE-8179

MULE-8179: NullPointerException thrown when using persistent object stor...
Updated gitignore to ignore factorypath
added travis-ci
skip tests.