ClientStorage constructor called with both wait=1 and
read_only_fallback=1 should return, indicating its readiness, when a
read-only connection was made.  This is done by calling
connect(sync=1).  Previously this waited for the ConnectThread to
finish, but that thread doesn't finish until it's made a read-write
connection, so a different mechanism is needed.

I ended up doing a major overhaul of the interfaces between
ClientStorage, ConnectionManager, ConnectThread/ConnectWrapper, and
even ManagedConnection.  Changes:

ClientStorage.py:

  ClientStorage:

  - testConnection() now returns just the preferred flag; stubs are
    cheap and I like to have the notifyConnected() signature be the
    same for clients and servers.

  - notifyConnected() now takes a connection (to match the signature
    of this method in StorageServer), and creates a new stub.  It also
    takes care of the reconnect business if the client was already
    connected, rather than the ClientManager.  It stores the
    connection as self._connection so it can close the previous one.
    This is also reset by notifyDisconnected().

zrpc/client.py:

  ConnectionManager:

  - Changed self.thread_lock into a condition variable.  It now also
    protects self.connection.  The condition is notified when
    self.connection is set to a non-None value in connect_done();
    connect(sync=1) waits for it.  The self.connected variable is no
    more; we test "self.connection is not None" instead.

  - Tried to made close() reentrant.  (There's a trick: you can't set
    self.connection to None, conn.close() ends up calling close_conn()
    which does this.)

  - Renamed notify_closed() to close_conn(), for symmetry with the
    StorageServer API.

  - Added an is_connected() method so ConnectThread.try_connect()
    doesn't have to dig inside the manager's guts to find out if the
    manager is connected (important for the disposition of fallback
    wrappers).

  ConnectThread and ConnectWrapper:

  - Follow above changes in the ClientStorage and ConnectionManager
    APIs: don't close the manager's connection when reconnecting, but
    leave that up to notifyConnected(); ConnectWrapper no longer
    manages the stub.

  - ConnectWrapper sets self.sock to None once it's created a
    ManagedConnection -- from there on the connection is is charge of
    closing the socket.

zrpc/connection.py:

  ManagedServerConnection:

  - Changed the order in which close() calls things; super_close()
    should be last.

  ManagedConnection:

  - Ditto, and call the manager's close_conn() instead of
    notify_closed().

tests/testZEO.py:

  - In checkReconnectSwitch(), we can now open the client storage with
    wait=1 and read_only_fallback=1.