vmq_reg_trie: Use genserver instead of genserver2 for improved memory management

[mths1]

Proposed Changes

We experience from time to time high memory consumption in vmq_reg_trie which can be traced back to genserver2 code. There seems to be no real advantage of genserver2.

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[ioolkos]

@mths1 quick question, though. vmq_reg_ordered_trie was supposed to improve the memory usage. Just wondering: Are there other reason you don't use that module?

[mths1]

@ioolkos : I rebased this change. There is no particular reason other than this would improve memory usage more or less for free also for the standard reg_trie. I could not measure any advantage of using genserver2... Let me know if you want to consider merging, otherwise I can also close this PR.

[ioolkos]

@mths1 let's keep it open until I'm able to run some sort of bigger test. I want to be sure I understand that there's no change for the handle_event logic.

[codeadict]

genserver2 buffers messages and improves selective receives at the cost of more memory. Probably need to dig back in history why it was used instead of regular gen_server