Remove Poll::deregister call and improve reconnect logic
Remove Poll::deregister call and improve reconnect logic

This PR is intended to fix the panic seen in #56 and makes a few changes
in the io module to that effect. The first is to remove the call to
Poll::deregister as the Mio docs indicate that it should not be required
(see comment in the issue mentioned above for link to docs). The second
change is that any reads or writes that return 0 affected bytes should
trigger a reconnect as the logging already notes that this means the
connection has been lost. In a similar vein, if a read or write
operation returns an io::error of ErrorKind::WouldBlock, we do not need
to reconnect as it is a spurious wake up. Lastly, it seemed like we
should be resetting the session timeout timer upon reconnect to avoid
potential races where the reconnect happens and the session timeout timer
fires before any operations can be made to prevent that timeout.

bonifaido/rust-zookeeper,bonifaido/rust-zookeeper