config: Add configuration to control GOMAXPROCS [backport to 18.8]

Eric Jurequested to merge
eju/backport-18-8/7092_packjobjecthook_spawn_execssive_threads into 18-8-stable

Add a new configuration field PackObjectHookMaxProc to the hooks section of the Gitaly configuration. This field defines the value of the GOMAXPROCS environment variable when invoking git-upload-pack(1).

The GOMAXPROCS value will be inherited by the git pack-objects hook process, allowing us to limit the Go scheduler parallelism and reduce the resources consumed by the hook.

Backport of !8511 (merged) to 18.9 Related to #7092 (closed)

Edited by Eric Ju

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