Borg 2.0

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Development Goals

Encryption: get rid of AES-CTR mode and use "session keys". This is mainly motivated by:
1. With AES-CTR and a global AES + MAC key one has to track the counter values perfectly to avoid leaking XORed plaintexts. This means extra synchronization and complexity between clients (and in a multi-threaded environment, between threads).
2. Using separate "session keys" in multiple threads makes multi-threading easier. Also, one can just start from 0 for the IVs of each session key.
3. Data encrypted using "session keys" could be more easily moved between repositories, which would allow efficient replication. This is not possible with a global CTR, since it may never repeat
4. Using "session keys" could allow to freely choose from a set of encryption algorithms -one algorithm is not the best for all machines.
use more modern / faster AEAD ciphers: AES-OCB and chacha20-poly1305
use a more modern KDF: argon2
concept of related repositories: using same chunker secret and id-hash key material.
borg transfer: enables copying archives from one repo to another related repository.
have separate encrypted metadata and data in a repo object
better and more symmetric way to deal with hardlinks: hlid
clean up the CLI
in general: get rid of legacy