Replace Dashmap with scc #543

Job	Run time
Run unit tests (ubuntu-latest)	1m 35s
Check clippy	21s
Run integration tests (5)	35s
Check formatting	14s
Compile on MSRV	18s
Run unit tests (macOS-latest)	1m 3s
Validate MSRV and minimal dependency versions (Cargo.lock.msrv, MSRV)	15s
Validate MSRV and minimal dependency versions (Cargo.lock.min, minimal dependency versions)	17s
Run unit tests (windows-latest)	4m 48s
Run integration tests (4.4)	38s
Release	0s
	10m 4s

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