Add loom simulations of spfs clean data race

    $ cargo test --bin clean-loom

This shows three variations on how `spfs clean` can race with writers to
a repo.

The first one tries to simulate the existing behavior and the
test fails as expected.

The other two demonstrate how a write-ahead log might prevent races. The
first one requires the write-ahead log to be locked while reading in all
the tags, which in turn would block writes for a significant amount of
time.

The second one proposes how the write-ahead log could be pruned over
time with a kakfa-like tombstone concept, and only requires the log to
be locked while actually deleting files.

In either case, the time while the write-ahead log is locked could early
exit in order to not block writers for an extended amount of time. In
this way it could make incremental progress by deleting some number of
objects in each pass.

Signed-off-by: J Robert Ray <jrray@jrray.org>

Cargo.toml

@@ -19,7 +19,7 @@ members = [
     "crates/spk-solve",
     "crates/spk-solve/crates/*",
     "crates/spk-storage",
-    "crates/spk",
+    "crates/spk", "clean-loom",
 ]
 resolver = "2"
 

clean-loom/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "clean-loom"
+authors.workspace = true
+edition.workspace = true
+version.workspace = true
+
+[dependencies]
+loom = "0.7"
+
+[lints]
+workspace = true