If we verify that the size matches reality, we can then expect to use
the filesystem to store the piece size as used in the signed PieceHash
from the uplink. Otherwise, the uplink might send a garbage size value,
leaving the storagenode with no good way to verify the uplink signature
on the piece at a later date.
Also fix the code in uplink/piecestore/ so that it sends a valid size,
because it was being rude and sending 0.
What: add monkit.Task to a bunch of functions that are missing it
Why: this will significantly help our instrumentation, data collection, and tracing about what's going on in the network
