Fix memory corruption in moduleHandleBlockedClients

By using a "circular BRPOPLPUSH"-like scenario it was
possible the get the same client on db->blocking_keys
twice (See comment in moduleTryServeClientBlockedOnKey)

The fix was actually already implememnted in
moduleTryServeClientBlockedOnKey but it had a bug:
the funxction should return 0 or 1 (not OK or ERR)

Other changes:
1. Added two commands to blockonkeys.c test module (To
   reproduce the case described above)
2. Simplify blockonkeys.c in order to make testing easier
3. cast raxSize() to avoid warning with format spec
