Parallel test runs sometimes write over a node #130

Sometimes when running parallel test methods, if a node is created at the same time only one will be displayed in the report. the method com.aventstack.extentreports.ExtentTest#createNode(java.lang.String) is not sync like the method com.aventstack.extentreports.ReportObservable#saveTest

I've created the PR #129 with some changes. I've ran some dozen of tests and report came out fine with that fix.