ISPN-2771 WriteSkewTest.testWriteSkewWithOnlyPut fails randomly

* Removed write skew check from put operation. Now, the write skew check
it is only performed during prepare time for local or clustered mode.

* Adapt the local mode tests to support this modification

Also, I've replaced all the assert keywords to assertEquals.

https://issues.jboss.org/browse/ISPN-2771