Refactor the Engine with stronger guarantees

The new Engine can allocate subnets and IPs because it introduces an explicit abstraction for shared (allocatable) resources, and plugs it into the CreateGroup API.

VMProvider and NetworkProvider have been abstracted behind interfaces to allow for testing of the Engine itself.

added 8 commits

• a2c715f1...863f1251 - 4 commits from branch master
• 0c488e4c - Refactor the Engine with stronger guarantees
• be0fcbde - Add a memory allocation test
• 46abd9ab - Improve logging messages on group termination
• 05853ec8 - Tune linter configuration

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added 2 commits

• 9c1ac9bc - Fix lint warnings
• 44d2d719 - Fix lint warnings

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added 1 commit

• d1a79e49 - Split buildGroupFromRequest() for readability

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