- Move from try/finally to with

- Start phasing out the _lock_acquire/_lock_release shortcuts

- Replace simpler @locked decorators with with statements.

  I've come to prefer the with style tp the decorator style.
  I think it's clearer.

  (The decorators preceeded the Python with statement.)

  I left the decorators in cases where they were used with preconditions.

This is mostly pretty mechanical, although it got a little delicate in
places and ... tests