Cache for RPC calls is actually kinda broken

[msooseth]

The cache is not coherent between different threads. So each thread will do another RPC call. I'm talking about this:

data VM (t :: VMType) s = VM
  { result         :: Maybe (VMResult t s)
  , state          :: FrameState t s
  , frames         :: [Frame t s]
  , env            :: Env
  , block          :: Block
  , tx             :: TxState
  , logs           :: [Expr Log]
  , traces         :: Zipper.TreePos Zipper.Empty Trace
  , cache          :: Cache
...

I discovered this when working on #581 -- it kept RPC fetching address 0 repeatedly (which is another issue related to the code being --bin-runtime and not --bin, because we don't yet support the --bin with parameters). Anyway, it should only have fetched the code of Address 0 once, not many of times for the many of threads.