Give option to set thread context class loader in BaseImpalaServlet

Applies as of r4049

Normally, subclasses of BaseImpalaServlet should need to set the thread
context class loader as they are typically working of the Spring MVC
architecture which handles class loaders properly.

However, this does not apply for Spring web flow, which creates new
application context for flow without setting the class loader properly,
implicitly using the thread context class loader.

Hence, need to add an option BaseImpalaServlet which will allow you to set
as a WEB-INF/web.xml servlet parameter whether to set the thread context
class loader.

Original issue reported on code.google.com by philzoio...@googlemail.com on 15 May 2009 at 10:03

pzoio / impala

Give option to set thread context class loader in BaseImpalaServlet #205