When a storagenode begins to run low on capacity, we want to notify
the satellite before completely running out of space. To achieve this,
at the end of an upload request, the SN checks if its available space has
fallen below a certain threshold. If so, trigger a notification to the
satellites.
The new NotifyLowDisk method on the monitor chore is implemented using the
common/syn2.Cooldown type, which allows us to execute contact only once
within a given timeframe; avoiding hammering the satellites with requests.
This PR contains changes to the storagenode/contact package, namely moving
methods involving the actual satellite communication out of Chore and into
Service. This allows us to ping satellites from the monitor chore
Change-Id: I668455748cdc6741291b61130d8ef9feece86458
7244a6a84e