Support modusDB instance per data directory

[jairad26]

This PR adds support for multiple modusDB instances in a single process, as long as they each reference their own dataDirectory. This moves the dataDirectory field out of the config and into the DB layer, and removes the validate function & WithDataDir functions, since DataDir is a mandatory field, it is now moved into an argument on the New() function.

[mangalaman93]

@jairad26 could you add a test for this? Because Dgraph uses global variables, I am not sure how this is working!

[mangalaman93]

This test seems to fail because we have global variables in Dgraph repo

func TestMultipleDBsDifferentDir(t *testing.T) {
	dataDir := t.TempDir()
	dataDir2 := t.TempDir()

	db1, err := modusdb.New(modusdb.NewDefaultConfig(), dataDir)
	require.NoError(t, err)
	defer func() { db1.Close() }()

	db2, err := modusdb.New(modusdb.NewDefaultConfig(), dataDir2)
	require.NoError(t, err)
	defer func() { db2.Close() }()

	_, err = db1.Mutate(context.Background(), []*api.Mutation{
		{
			Set: []*api.NQuad{
				{
					Namespace:   0,
					Subject:     "_:aman",
					Predicate:   "name",
					ObjectValue: &api.Value{Val: &api.Value_StrVal{StrVal: "A"}},
				},
			},
		},
	})
	require.NoError(t, err)

	_, err = db2.Mutate(context.Background(), []*api.Mutation{
		{
			Set: []*api.NQuad{
				{
					Namespace:   0,
					Subject:     "_:aman",
					Predicate:   "name",
					ObjectValue: &api.Value{Val: &api.Value_StrVal{StrVal: "B"}},
				},
			},
		},
	})
	require.NoError(t, err)

	query := `{
			me(func: has(name)) {
				name
			}
		}`
	qresp, err := db1.Query(context.Background(), query)
	require.NoError(t, err)
	require.JSONEq(t, `{"me":[{"name":"A"}]}`, string(qresp.GetJson()))

	// drop db1
	require.NoError(t, db1.DropAll(context.Background()))
}