Sqlite: add more locking to DBLayer methods

[rvl]

Relates to issue #154.

It seems to be best to put a big lock around all operations which use the SQLite connection.

I was getting the following failed test:

  λ> quickCheck (prop_parallel db)
  *** Failed! Exception: 'SQLite3 returned ErrorError while attempting to perform step: not an error' (after 5 tests and 3 shrinks):
  ParallelCommands
    { prefix = Commands { unCommands = [] }
    , suffixes =
        [ Pair
            { proj1 =
                Commands
                  { unCommands =
                      [ Command
                          (At (ReadTxHistory (Val (MWid "c"))))
                          (At (Resp (Right (TxHistory (fromList [])))))
                          []
                      ]
                  }
            , proj2 =
                Commands
                  { unCommands =
                      [ Command
                          (At (PutTxHistory (Val (MWid "a")) (fromList [])))
                          (At (Resp (Left (NoSuchWallet (Reference (Symbolic (Var 0)))))))
                          [ Var 0 ]
                      ]
                  }
            }
        ]
    }