Track number of unique stack traces/locations/mappings/functions seen

[brancz]

As eluded to in #284, we are likely going to want more search functionalities in Parca, the cardinality of this is important to understand as it can make the difference between being able to use a uint64 as the ID or having to use something with a larger addressable space. The difference between being able and not being able to use uint64 means the difference of being able to use or not use the roaring bitmaps in such a search functionality.

Somewhat paradoxically the max value of a uint64 seems very high, but since it is the combinatoric possibilities of locations, it doesn't seem impossible to hit the limit. We should understand the real-world behavior in more detail.