This commit adds a basic itest that is run under the `test_native_sql`
flag. It tests that the migration is idempotent and that the node cant
be restarted without the `db.use-native-sql` flag once the graph
migration has been run.
This commit plugs in the graph kvdb-to-sql migration for builds
containing the `test_native_sql` tag. This will allow us to perform
local tests and write itests for the migration without exposing it to
the production release build.
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.
