Depends on D4320
A couple of changes in the send log logic:
- we separate the `value` and the `blobHash`. Until now, we only used the member `value` that could really be either of these things, but now it is split into two separate fields so it's easier to understand.
- the logic of the `readRequest` is simplified
- fixed the bug in `if else` in `terminateCallback`
- checking the real item size against the dynamo DB limit
I realize this should be split into the separate diffs, I'm going to do this if we accept the high-level idea.