* feat(snapshots): improved performance when uploading huge files
This is controlled by an upload policy which specifies the size
threshold above which indvidual files are uploaded in parts
and concatenated.
This allows multiple threads to run splitting, hashing, compression
and encryption in parallel, which was previously only possible across
multiple files, but not when a single file was being uploaded.
The default is 2GiB for now, so this feature only kicks in for very
larger files. In the future we may lower this.
Benchmark involved uploading a single 42.1 GB file which was a VM disk
snapshot of fresh Ubuntu installation (fresh EXT4 partition with lots
of zero bytes) to a brand-new filesystem repository on local SSD of
M1 Pro Macbook Pro 2021.
* before: 59-63s (~700 MB/s)
* after: 15-17s (~2.6 GB/s)
* additional test to ensure files are really e2e readable
This is a safety measure which addresses P0 improvement for #1732.
Given that retention policies that retain nothing make no sense, this
is not considered a breaking change.
Added policy.Definition which allows us to precisely report where
each piece of policy came from.
Fixed a one-off bug with "noParent", which prevented merging of parent
policies one level too soon.
Added a whole bunch of merging helpers and generic reflection-based
test that ensures every single merge is tested.
