Add mimalloc allocator and adjust message save threshold #1361

Job	Run time
sanity / Check	3m 4s
sanity / Clippy	3m 31s
sanity / Test documentation	4m 2s
sanity / Validate commit messages	1s
sanity / Rustfmt	4s
sanity / Sort dependencies	30s
sanity / Unused dependencies	49s
build and test macOS-aarch64	2m 57s
build and test Linux-x86_64-musl	5m 51s
build Windows-x86_64	5m 30s
backwards_compatibility / check_commit_message	3s
backwards_compatibility / build_and_test	1m 39s
finalize_pr	1s
	28m 2s

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