MSI: Slight refactoring

[naydav]

Internal build
https://bamboo.corp.magento.com/browse/M2-AT2449-1

[maghamed]

nice changes, all of them make sense for me.

What I propose to add is re-naming of SourceItemSaveInterface -> SourceItemsSaveInterface
as this service provides a possibility of saving Multi SourceItems per call, so that would be more semantically friendly to call it SourceItemsSaveInterface in plural form

[magento-cicd2]

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[naydav]

Internal static build (INTEGRITY-SANITY-EE) is red due to MAGETWO-71596