attempt to do better padding on pieces being written into sectors

[whyrusleeping]

No description provided.

[ribasushi]

Relevant PR's to make this all work:

diff --git a/go.mod b/go.mod
index 503e95c5d..2d2548ccc 100644
--- a/go.mod
+++ b/go.mod
@@ -31 +31 @@ require (
-       github.com/filecoin-project/go-commp-utils v0.0.0-20201119054358-b88f7a96a434
+       github.com/filecoin-project/go-commp-utils v0.1.1-0.20210407220142-9d7f7aaf8c9c
@@ -35 +35 @@ require (
-       github.com/filecoin-project/go-fil-markets v1.1.9
+       github.com/filecoin-project/go-fil-markets v1.1.10-0.20210407195421-65658e33ed64

filecoin-project/go-fil-markets#536
filecoin-project/go-commp-utils#3

[ribasushi]

This has been tested extensively on nerpanet:

• Storage of an 4x artificially padded data filling entire sector
• Storage of a 2x artificially padded data filling half of sector, with the other half being auto-filled by CC
• Retrieval from a separate node from the saved-by-default unsealed copy
• Retrieval from a separate node while triggering unsealing on the miner

[magik6k]

Some tests would be nice (sealer_test.go, should be easy enough to set them up)

[magik6k]

1 Nit