BUGFIX: WATCH + MGET clearing inTransaction prematurely. WATCH + HGETALL does not use post_proc

The previous commit didn't fix all bugs. Hopefully this will make things right.

To reproduce the 1st problem:

  > t = yield conn.watch(k)
  > t.mget([k])
  > assert t.inTransaction # boOoOm!

To reproduce the second problem:

  > t = yield conn.watch(k)
  > h = yield t.hgetall(k)
  > assert type(h) == dict # boOoOm!

This commit introduces a `commit_cc` function (similar to
`unwatch_cc`) which clears the `inTransaction` flag only if there has
been a `MULTI` command before (fixes the 1st problem).

The second problem is fixed by not delaying using `post_proc`
function if only `WATCH` has been issued.