worker: add worker.getMemoryUsage() API #1011

Job	Run time
Build slim tarball	1m 36s
aarch64-linux: with shared libraries	29m 24s
x86_64-darwin: with shared libraries	1h 4m 3s
aarch64-darwin: with shared libraries	37m 6s
x86_64-linux: with shared libraries	34m 35s
	2h 46m 44s

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