GEODE-8756: Fix CacheableString::objectSize

[gaussianrecurrence]

• objectSize was not taking into account null-terminator character,
  which as of C++11, as stated in § 21.4.7.1, should be included at
  the end of the same string character sequence.
• Also, in those cases in which SSO applied, the returned size was
  higher than the actual one.
• A parametrized UT was added to check objectSize is returning the
  right size. Note that no fixed sizes are part of such test, given
  that each std::string implementation could have a different SSO
  size.
• Added a new class called size_tracking_allocator, which is a custom
  allocator used to track the size STL objects.

[gaussianrecurrence]

As STL objects depends directly on which specific implementation is choosen, and there is no standarization regarding it, sadly there is no way to guarantee that introducing this change won't blow up in the future. Closing the PR