MemPool the debug output stream buffers

MemPool the debug output stream buffers

The CurrentDebug output stream controller for cache.log was
defined as a std::ostringstream object and allocated with
new/delete on each call to debugs().

The std::ostringstream is defined as a templates output stream
which uses the std::allocator<char> built into libc when its
new()'d. Since this is all internal to the STL library
definitions it links against the libc global-scope allocator.

However, there is no matching deallocator definition and when
the object is delete()'d the standard C++ operator overloading
rules make the global-scope SquidNew.h definition of
::operator delete() be the method of deallocation. That uses
free() internally.

To resolve the mismatch of new()/free() we must define a
wrapper class with explicit class-scope new/delete operators
instead of relying on weak linkages to overloaded global scope
operators.

As a result the memory is new()'d and free()'d. As detected by
Valgrind