Report replication progress as a float

[joonas-fi]

https://github.com/function61/varasto/blob/ea0be71a1026a720be1a1c265ff0d18c1838907d/pkg/stoserver/storeplication/replicationcontroller.go#L143

Truncating to integer is needlessly coarse, resulting in steps in the metrics:

The larger backlog of transfers one has, the more this issue is amplified.

[joonas-fi]

Also while at it, replace 65536.0 with the appropriate constant