Create/overwrite support

Phases:

1. Create a CHK file cap from a local file. 4 weeks.
   • Simple server selection (ask them about space, spread around).
   • Slot creation.
   • Streaming encoding and upload.
   • UI.
2. Create a CHK dir cap from a local directory. 2 weeks.
   • Just encode first level directory listing and use the above, recursing.
   • UI.
3. Create a SDMF file cap from a local file. 3 weeks.
   • It's a different encoding, otherwise similar to 1.
   • More PK cryptography.
   • Existing UI must now choose format.
4. Create a SDMF dir cap (plain folder) from a local directory. <1 week.
   • This is likely to come free by virtue of 2 and 3.
5. Create new MF: 3 weeks.
   • Build the structure with one snapshot and upload it using the above. No DAG.
   • Existing UI must now choose plain or magic.
6. Overwrite a SDMF cap. 2 weeks.
   • Uses the above.
   • Needs new code to parse and decrypt some write-auth stuff in the existing cap.
7. Edit existing MF without maintaining the DAG.
   • Just add a snapshot.
8. Maintain the MF DAG when adding versions.
   • Create a DAG arc from the version that was modified.
9. Resolve conflicts in MF DAG.
   • Needs lots of UI.