TACHYON-120 Fail to cast String to other objects for JsonObject

Currently JsonObject.get just directly cast parameter Object to the generic T type. 
This could generate class cast exception such as the one from String to DependencyType.

Modify the code to use Jackson JSON formatter instead by passing the Class<T> type in the get method to avoid Java type erasure to get the actual parameterized type.
unsafe cast.  if the value is of the type, you can avoid the convert.  if not the exception will mean something real.