Allow connections to be re-established after closing them.

Prior to this change connections could not be re-established after
closing them. That caused a problem when using beaneater in prefork
like servers.

For example, when running Unicorn with preload_app set to true.
Connections must be re-established after workers have been forked.

With this commit the Beaneater::Pool can be closed causing connections
to automatically be lazily re-established.

This also has the side benefit of allowing calling code to recover
from connection errors. Upon a connection error the calling code need
only to close the pool and try again.