Rename rdf keyword

[andyfengHKU]

• Rename RDF table names for simplicity.
• Rename RDF literal table iri column to val column so that it doesn't conflict with resource table iri column.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4ff03b4) 93.25% compared to head (0fab0fe) 93.26%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2618      +/-   ##
==========================================
+ Coverage   93.25%   93.26%   +0.01%     
==========================================
  Files        1037     1037              
  Lines       38870    38870              
==========================================
+ Hits        36247    36251       +4     
+ Misses       2623     2619       -4     

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