Use an external siad

[ro-tex]

SiaStream comes with a built-in Sia node. For users who already run their own node for other purposes it would be node to be able to reuse that node without spending extra resources on an additional one.

It is currently possible to run the image in a way which would make the stand-alone external Sia daemon available to SiaStream but that does not prevent SiaStream from running its own built-in node. This node will then start to sync the blockchain, resulting in extra system load which is highly undesirable.

This PR can only be implemented after some changes in SiaStream which would make it possible for it to not start its own node if it detects an already running external node.

Note:
Users can already map their node's storage directory to SiaStresam's image via a volume and thus use the same Sia data directory. The problem with that is that only one Sia node can be using the directory at the same time, so the user needs to be careful to stop their other Sia node before running SiaStream. This is not ideal.