Support for tracking application memory usage by category

[mbautin]

It looks like it would be possible to allow applications to keep track of their memory usage by application-specific categories using the following approach:

• The application calls a tcmalloc extension function that sets a thread-local "tracking tag" (let's say an int32 or int64).
• Malloc calls use the current value of the tracking tag and propagate it to page heap allocation functions. The span metadata records get tagged with the tracking tag.
• Through another new tcmalloc extension function, the application can request page-level statistics by the app-specific tag.