Support Single Column PRIMARY KEY for nodes

[ray6080]

To add the primary key constraint for nodes, following things need to be done:

• introduce the keyword in the loader, and enable the loading of primary key column into storage as a regular node property. (See Add the concept of primary key for nodes into loader #215)
• the data type of primary key property should be extended to INT64 if we add it later. Currently, we treat all primary key values as string in the loader.
• an index (hash index) should be added/used to enforce the constraint of uniqueness.

[semihsalihoglu-uw]

So do we still treat all primary key values as strings? Or do we support string and int64s?

[ray6080]

So do we still treat all primary key values as strings? Or do we support string and int64s?

Currently, in the loader, we still treat all primary key values as strings.

[ray6080]

I believe this is already done now.