Tweak some table column storage parameters
Introduce PendingAsset, created before transcoding

This changes the order of transcoding table update from:
  - insert transcode, wait for completion
  - insert asset on completion
  - update slot_asset
to:
  - insert asset, no sha1
  - update slot_asset
  - insert transcode, wait for completion
  - update asset.sha1 on completion

Also add asset.size column.
Update front-end for pending asset change
Synchronize schema
Fix transcode job lookup; report out collection errors