Fix unbounded memory growth with perfmaps/JIT dumps

[brancz]

Why?

We have seen unbounded growth of memory in customer environments in JITed scenarios where perfmaps/jitdumps are used.

What?

The memory usage of this code path is entirely held by newStringConverter and Deduplicate: https://pprof.me/c801111f0576cba2f7dc3feb77ad7008/?profileType=profile%3Ainuse_space%3Abytes%3Aspace%3Abytes&filter_by_function=github.com%2Fparca-dev%2Fparca-agent%2Fpkg%2Fperf.ReadPerfMap&search_string=github.com%2Fparca-dev%2Fparca-agent%2Fpkg%2Fperf.ReadPerfMap

How?

This patch replaces the string converter with a string dictionary to reduce memory usage caused by duplicate strings, and compacts it together with the deduplicated perfmap entries, which previously were holding however many entries were in the perfmap, even if the majority was deduplicated.

Test Plan

Unit tests are passing and we ultimately have to test this in customer environments as it can only be observed in long-running environments.

Further notes

If this is not good enough, ultimately the perfmap should be read from the bottom up to never even load "outdated" entries in the first place, I'd only want to implement that though if this doesn't already fix things.

If do do go down that route these two would be helpful:

• rscanner to scan the perfmap from bottom to top
• Interval tree to keep track of memory ranges