In order to be compliant with tcmalloc's extension ownership
determination function, we have to expose a function that will
say if a chunk was allocated by us.
As to whether or not this has security consequences: someone
able to call this function repeatedly could use it to determine
secrets (cookie) or craft a valid header. So this should not be
exposed directly to untrusted user input.
Add related tests.
Additionally clang-format caught a few things to change.