remove nats resubscribe/reconnect logic

The on_reconnect callback is actually called when a reconnect ATTEMPT is
made, not when it reconnects. The NATS client resubscribes for us
already, so we don't need to use it anyway.

[#54338464]

Signed-off-by: Glenn Oppegard <goppegard@pivotallabs.com>
bump nats dependency to 0.5.x for cluster support

Signed-off-by: Eric Malm <emalm@pivotallabs.com>