Add systemd dependency on clickhouse-server

[lamaral]

Without this dependency, graphite-ch-optimizer will try to start before the Clickhouse server is ready, leading to a failure to start.

[lamaral]

My concern was the situation when the service works remotely. But it looks like it won't be an issue

According to my research, this should not be a problem. Wants and After will happily ignore non-existing units. I also tried it on an existing server and the service started fine, even with the other unit not existing.

[Felixoid]

Just in case, it makes sense to update the golang version too. 1.15 is a bit outdated

[Hipska]

@Felixoid good idea, a fresh build would not hurt anything 😉

[Felixoid]

@lamaral are you here? =)