Refactor "merged callstacks profiler" with new stack traces datastructure

[Bluezen]

I have created ParallelStacksProfiler, a quick clone of MergedCallStacksProfiler to test the new stack trace datastructure. Instead of refactoring MergedCallStacksProfiler I wanted to compare both implementation and output side by side.

TreeNode code is very easy to understand, it will certainly be simpler to maintain and to adapt. I really like the idea of using HashMap property to aggregate call stacks, so much cleaner! Overall I think we could simply trash the old implementation. Good job!!

A few remarks though:

• To print stacks from top to bottom I still have to do a revert (line 132) as the do_stack_snapshot method gives us methods_ids as a stack (to read from bottom to top)... if you think the prefered way to read stacks are the other way around maybe we should do the revert in the build_from_sequences tree method
• we loose the thread_ids... i know we don't make any use of them right now, so not a big deal at the moment, but I wanted to point it out
• depth in TreeNode can be discarded I think

[Bluezen]

We don't have to merge this PR, I pushed it in case you wanted to have a look at how MergedCallStacksProfiler could look like with TreeNode... but something tells me you already have a more generic approach to print TreeNode and maybe already refactored both MergedCallStacksProfiler and CpuHotpathProfiler XD

[ogxd]

Nooo haha I just did this, so I was really happy to see your PR. Didn't had time to check it yet however

[ogxd]

To print stacks from top to bottom I still have to do a revert (line 132) as the do_stack_snapshot method gives us methods_ids as a stack (to read from bottom to top)... if you think the prefered way to read stacks are the other way around maybe we should do the revert in the build_from_sequences tree method

Ah yesss didn't thought of that. I think you put it in the best place. Maybe the code to rebuild all sequences but reverted can be compacted a bit, will see if I can get something

we loose the thread_ids... i know we don't make any use of them right now, so not a big deal at the moment, but I wanted to point it out

Yeah true.... Made some changes so that it is a little more flexible: #58

depth in TreeNode can be discarded I think

Ok !

[Bluezen]

merged call stacks profiler has been replaced by pstacks profiler that make use of the new tree structure. The output is exactly the same than before. Unfortunately I haven't yet been able to refactor the rendering of the tree and make it available to other profiler... it's a bit tricky