Switch from datastore files to namespace usage

[hickeng]

For 6.0 and up, we may be able switch to using namespaces for the following:

• session logging
• kv store

Doing so would make logs a little hard to access out-of-band, but would move us closer to not needing datastore upload/download privileges, and away from the serial port issues:

• limited bandwidth
• comparatively high cpu utilization
• no easy way to multiplex datastreams

Questions:

• how are namespaces created while VM is live
• how are namespaces identified and accessed from guest
• is there a viable way to append data
• what are the storage limitations
• what are the visibility options (guest only vs API only vs both?)

Follow up:

• [ ] do we see the same failure to set started field during concurrent exec as noted in https://github.com/vmware/vic/issues/6700#issuecomment-419995874 - the workaround mentioned in that comment will need removing to make the issue visible, or a specific stress test added.

Details:

• VM namespace manager is found via this vmodl chain: ServiceContext->retrieveInternalContent->vm-namespace-manager
• Capability is NamespaceDB
• Namespaces are accessed from the guest via the namespacetool
• Namespaces are purged on snapshot. bug1823451 is open to add option to suppress this behaviour.

[hickeng]

@caglar10ur @fdawg4l potentially significant question when it comes to implementing exec, and for cleaner structure in the appliance.

[fdawg4l]

I don't know what "namespaces" means in this context. Please elaborate.