Drop obsolete class StringArea

class StringArea was always intended to only be a temporary measure
before SBuf was completed and merged. That has now happened.
Cleanup: convert helper allocation from cbdataAlloc to new

The helper_server and helper_stateful_server classes are already 
CBDATA_CLASS2 defined classes with new/delete operators and the free
operation already handled by delete(). However teh allocation was not
using new() and thus not calling the class constructors properly.
Implement SBuf and MemBlob size distribution stats