kvdb/postgres: separate general sql code from postgres

In this commit, changes are made to the `kvdb/postgres` package so that
all all the non-postgres-specific code is generalised to be applicable
for all sql code. A follow up commit will move all the general sql code
into its own package.

kvdb/postgres/db_test.go

@@ -41,7 +41,7 @@ func TestPanic(t *testing.T) {
 	f, err := NewFixture("")
 	require.NoError(t, err)
 
-	err = f.Db.(*db).Update(func(tx walletdb.ReadWriteTx) error {
+	err = f.Db.Update(func(tx walletdb.ReadWriteTx) error {
 		bucket, err := tx.CreateTopLevelBucket([]byte("test"))
 		require.NoError(t, err)