helium: use a lock to make DeferredCommitBuffer thread safe #691

Job	Run time
build-windows (windows-latest, Release)	16m 58s
build-linux (ubuntu-20.04, Release)	10m 31s
build-windows (windows-latest, Debug)	4m 57s
build-linux (ubuntu-22.04, Release)	9m 25s
build-linux (ubuntu-22.04, Debug)	16m 36s
build-linux (ubuntu-24.04, Release)	10m 16s
build-linux (ubuntu-24.04, Debug)	16m 57s
build-macos (macos-latest, Release)	7m 54s
	1h 33m 34s

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