CXX-306 TCMalloc is not appropriate for driver library builds
CXX-307 Don't check error codes on 26compat branch

On the 26compat branch, we trust upstream (server) to provide
correctly curated error codes.
CXX-274 Backport server r2.6.3..r2.6.4 changes
Merge branch '26compat' of github.com:mongodb/mongo-cxx-driver into 26compat