rearrange encode/upload, add URIs, switch to ReplicatingEncoder

rearrange encode/upload, add URIs, switch to ReplicatingEncoder

Added metadata to the bucket store, which is used to hold the share number
(but the bucket doesn't know that, it just gets a string).

Modified the codec interfaces a bit.

Try to pass around URIs to/from download/upload instead of verifierids.
URI format is still in flux.

Change the current (primitive) file encoder to use a ReplicatingEncoder
because it provides ICodecEncoder. We will be moving to the (less primitive)
file encoder (currently in allmydata.encode_new) eventually, but for now
this change lets us test out PyRS or zooko's upcoming C-based RS codec in
something larger than a single unit test. This primitive file encoder only
uses a single segment, and has no merkle trees.

Also added allmydata.util.deferredutil for a DeferredList wrapper that
errbacks (but only when all component Deferreds have fired) if there were
any errors, which unfortunately is not a behavior available from the standard
DeferredList.