Ability to have true write-only repos

[verygreen]

Currently borg cannot implement true write-only repos, the client needs to read back manifest, add new archives to it and write it back. Server-side acting as a mostly dumb key/value store.

A desirable (in some scenarios) feature is to have borg protocol (and possibly disk format for borg store) allow a session that goes like:
start session
archive create XXX
write block 1
...
write block N
archive commit XXX
end session.

This probably should still allow reading of known blocks for cache rebuild purposes.

I imagine it's possible to have a shim layer that would reuse same disk format, but will add more of the session framework and let server update the manifest locally by client instruction and client still sending data blocks as usual, thus allowing to have more "protocol modules" to talk to true key/value stores at cloud providers and such as desired.

[RonnyPfannschmidt]

its a considerable amount of restructuring needed to make the repo format write-only,

also there is no such thing as a "archive create/commit"

this needs quite some work to turn into something actionable

[ThomasWaldmann]

PR #8844.