David Turner aec44fecbc Decouple DiskThresholdMonitor & ClusterInfoService (#44105)
Today the `ClusterInfoService` requires the `DiskThresholdMonitor` at
construction time so that it can notify it when nodes report changes in their
disk usage, but this is awkward to construct: the `DiskThresholdMonitor`
requires a `RerouteService` which requires an `AllocationService` which comees
from the `ClusterModule` which requires the `ClusterInfoService`.

Today we break the cycle with a `LazilyInitializedRerouteService` which is
itself a little ugly. This commit replaces this with a more traditional
subject/observer relationship between the `ClusterInfoService` and the
`DiskThresholdMonitor`.
2019-07-09 18:43:32 +01:00
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