MINIFICPP-2504 MergeContent should be single threaded #5773

Job	Run time
macos-xcode	49m 20s
ubuntu-22.04-clang	14m 11s
rocky	15m 10s
Docker build for integration tests	34m 3s
ubuntu-24.04	37m 12s
windows-2022	47m 44s
Docker integration tests	24m 36s
	3h 42m 16s

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