fix(mutations): nullable payloads, rollback individual mutations on error

[benjie]

Wraps each mutation in a transaction, automatically rolling back so that a) results of previous mutations are not invalidated (see #75) and b) future mutations are not prevented.

Fixes graphile/crystal#563
Fixes #75

[benjie]

From graphile/crystal#563 (comment):

The error only happens with missing grants:

CREATE OR REPLACE FUNCTION public.test() RETURNS boolean LANGUAGE 'sql' AS $func$ SELECT true $func$;
REVOKE EXECUTE ON FUNCTION public.test() FROM PUBLIC;
mutation regression {
  test(input: {}) {
    clientMutationId
    boolean
  }
}
{
  "data": null, // <---
  "errors": [
    {
      "message": "permission denied for function test",
      "locations": [
        {
          "line": 29,
          "column": 3
        }
      ],
      "path": [
        "test"
      ]
    }
  ]
}