compose segment files locally, remove the need for smart servers

[RonnyPfannschmidt]

Adding 'enhancement' label based on user approval. Existing labels are kept for issue #191.

[ThomasWaldmann]

IIRC, the repo index and the usage of segments is an implementation detail of this k/v store implementation. if we had another backend for kv storage, it could work completely differently (aside from the basic kv operations).

[anarcat]

it would be great to have an abstraction for that kv storage... it would help in resolving #102 or generally to make borg extensible to more than the "borg server over SSH" backend. :)

we do have a "generic" k/v API, but the RemoteRepository implementation is expected to respond to way more stuff than just CRUD key/values... it actually has a higher-level implementation with commands like repair and check which is somewhat problematic if we deal with abstract storage backends...

[ThomasWaldmann]

#8332 solved half of this:

• a smart server is not needed any more, any kv store implemented as borgstore backend would do (currently there is sftp as a remote server-less backend)
• we do not compose segment files locally though, but rather store each chunk to a separate object in the store.