Update MySQL and Sqlite libraries to use new StatementCache mechanism

Author: iand675

persistent-mysql/Database/Persist/MySQL.hs

@@ -126,7 +126,7 @@ open' :: MySQL.ConnectInfo -> LogFunc -> IO SqlBackend
 open' ci logFunc = do
     conn <- MySQL.connect ci
     MySQLBase.autocommit conn False -- disable autocommit!
-    smap <- newIORef $ Map.empty
+    smap <- makeSimpleStatementCache
     return $ SqlBackend
         { connPrepare    = prepare' conn
         , connStmtMap    = smap

persistent-sqlite/Database/Persist/Sqlite.hs

@@ -266,7 +266,7 @@ wrapConnectionInfo connInfo conn logFunc = do
         Sqlite.reset conn stmt
         Sqlite.finalize stmt
 
-    smap <- newIORef $ Map.empty
+    smap <- makeSimpleStatementCache
     return $ SqlBackend
         { connPrepare = prepare' conn
         , connStmtMap = smap
@@ -455,7 +455,7 @@ migrate' allDefs getter val = do
 -- with the difference that an actual database isn't needed for it.
 mockMigration :: Migration -> IO ()
 mockMigration mig = do
-  smap <- newIORef $ Map.empty
+  smap <- makeSimpleStatementCache
   let sqlbackend = SqlBackend
                    { connPrepare = \_ -> do
                                      return Statement