nixos/elasticsearch: Wait for elasticsearch to start up properly
nixos/elasticsearch: Wait for elasticsearch to start up properly

Other services that depend on elasticsearch should be started after
it, but since the versions we're packaging have to run as
"Type=simple", they're started as soon as the elasticsearch binary has
been executed, likely winning the race against it.

This makes sure elasticsearch is up and running, responding to a
simple query, before dependents are started.

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