In this commit, a new `test_native_sql` build flag is defined. If this
build flag is used along with the `--use-native-sql` config option, then
the SQLStore implementation of the graphdb.V1Store will be initialised.
This is then used to run our itest suite against the new SQLStore graph
implementation.
NOTE that this only works for new nodes currently - the migration from
kv-to-sql is yet to be implemeneted.
