switch to the strict state monad

I had not realized what a memory leak the lazy state monad could be,
although I have not seen much evidence of actual leaking in git-annex.
However, if running git-annex on a great many files, this could matter.

The additional Utility.State.changeState adds even more strictness,
avoiding a problem I saw in github-backup where repeatedly modifying
state built up a huge pile of thunks.

Common.hs

@@ -3,7 +3,7 @@ module Common (module X) where
 import Control.Monad as X hiding (join)
 import Control.Monad.IfElse as X
 import Control.Applicative as X
-import Control.Monad.State as X (liftIO)
+import Control.Monad.State.Strict as X (liftIO)
 import Control.Exception.Extensible as X (IOException)
 
 import Data.Maybe as X