create ~server_name ~log get_address returns a persistent rpc connection to a
server whose host and port are obtained via get_address every time we try to
connect. For example, get_address might look up a server's host and port in
catalog at a particular path to which multiple redundant copies of a service are
publishing their location. If one copy dies, we get the address of the another one
when looking up the address afterwards.
All connection events (see the type above) are passed to the on_event callback, if
given. If a ~log is supplied then these events will be written there as well,
with a "persistent-connection-to" tag value of server_name, which should be the
name of the server we are connecting to.
`Failed_to_connect error and `Obtained_address addr events are only reported if
they are distinct from the most recent event of the same type that has taken place
since the most recent `Attempting_to_connect event.
connected returns the first available rpc connection from the time it is called.
When currently connected, the returned deferred is already determined.
If closed has been called, then the returned deferred is never determined.
close t closes the current connection and stops it from trying to reconnect.
After the deferred it returns becomes determined, the last connection has been
closed and no others will be attempted.
is_closed t returns true if close has ever been called, even if the returned
deferred has not yet been fulfilled.
close_finished t becomes determined at the same time as the result of the first
call to close. close_finished differs from close in that it does not have the
side effect of initiating a close.