Revert "Overriding "getCacheKey" in HystrixTaskHandler. This enables all task"

This reverts commit 515e9d28702011c5b2a0d15f14ffc348418a1d27.
Extended HystrixTaskHandler to create RequestCacheableHystrixTaskHandler.
This enables one to implement Hystrix based request caching.
This would also necessiate that HystrixRequestContext be initialized and
shutdown for each request that triggers the task handler. All
RequestCacheableHystrixTaskHandlers will be executed by
RequestCacheableTaskHandlerExecutor which extends
HystrixTaskHandlerExecutor. Made changes to
TaskHandlerExecutorRepository to return appropriate executor based on
the type of the TaskHandler.