feat(memory): add reserved_memory_bytes opt

[hzxa21]

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What's changed and what's your intention?

Related #16130

Prior to this PR, reserved memory for compute node is hardcoded to 30% * total available memory, which may be too much under certain workload and machine set-up. This PR adds a compute node opt for user to configure the reserved memory bytes explicitly, with the default behavior unchanged.

Checklist

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Release note

reserved_memory_bytes startup opt and RW_RESERVED_MEMORY_BYTES env are introduced for compute node to override the reserved memory configuration.

The default setup remains unchanged, which is compute node using 30% of its total memory as the reserved memory.

[lmatz]

LGTM, thanks