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Also upgrade the restic version used for tests to 0.9.10.

Support per-source excludes (for file handlers)

Closes #4

See merge request !3

Fixes issue #4.

The global HTTP handler overrides /debug.

We can keep track of how large the backups are in the metadb.

Adds the max_metadata_age_days parameter to the metadb configuration,
and runs a periodic thread to expire the old metadata.

Handle sqlite concurrency issues properly, retrying on BUSY
errors. Use proper transaction wrappers.

Snapshot cache

See merge request !2

This will eventually allow us to use --previous and skip the expensive
remote snapshot lookup.

Cache directories are sharded by source name, enabling re-use.