Fix APropertyTable.getDepth() NPE

APropertyTable.getDepth() would throw a NullPointerException because
DEPTH is a ThreadLocal static variable, but the static block which
initializes DEPTH to 0 is only executed by a single thread.  NPE
manifested itself when running in MultiThreaded mode with dataDriven
steps inside individual webTests.
Increase selftest memory limits

Bump the selftest memory lint from 128m to 256m
Fix WebTestPropertyHelper to be thread safe

When running in multithreaded mode, multiple webtest tasks defined
within the same test file could cause DynamicProperty expansion to stop
working as the tests start and finish.
Fix NullPointerException in ContextHelper

Prevent NullPointerException in ContextHelper.defineAsCurrentResponse
when called without an existing response in the context.
CSV Support in dataDriven tags

Support csv files in dataDriven tags
Support for xlsx

Support xlsx files in dataDriven and excel steps.