fix: use fedimint-aleph-bft

[maan2003]

fixes #4054

[maan2003]

I didn't use async BackupReader because we use Cursor<Vec<u8>> which is fine to read in async code.

[joschisan]

I didn't use async BackupReader because we use Cursor<Vec<u8>> which is fine to read in async code.

I think using BackupReader would allow for a cleaner implementation with a second UnitReader struct and two separate constructors instead of the load_session method that now returns both, but we can also leave it until we upstreamed the change.

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (3ab7dce) 58.37% compared to head (bb15908) 58.32%.

Files	Patch %	Lines
fedimint-server/src/atomic_broadcast/backup.rs	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4116      +/-   ##
==========================================
- Coverage   58.37%   58.32%   -0.06%     
==========================================
  Files         192      192              
  Lines       42659    42646      -13     
==========================================
- Hits        24904    24873      -31     
- Misses      17755    17773      +18     

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[maan2003]

two separate constructors instead of the load_session method

I attempted this, we still need to find initial units_index for UnitSaver, for which we have to check the entire old units