Feature/preserve connection pool

If we have only one ElasticSearch server in the cluster and it fails, the canonical behaviour of `pyes` is to remove it from the connection pool — hence interrupting the operation since _there are no servers left in the pool_.

This PR adds an optional parameter named `preserve_connection_pool` to the `ES` constructor.
If it is set to `True`, failing servers will **not** be removed from the connection pool, thus allowing retry attempts to be immediately made.