Add optional support for write locks

These write locks prevent accidental misuse of `Synchronizer`s by
establishing an advisory lock on the state file using the `flock`
system call. Correctly configured writers will check this lock to
prevent duplicate writers.

The new write locking feature is configured through a new builder
type, which provides an alternate way to configure `Synchronizer`
options. We can extend this pattern in the future.

Write locking functionality is gated behind the `write-lock` flag
and therefore has no performance impact when disabled. Benchmarks
show an ~1% increase in write latency when the feature is enabled
(the first write may experience additional latency as the lock is
acquired.)

.github/workflows/ci.yml

@@ -33,6 +33,9 @@ jobs:
       - name: Run cargo test
         run: cargo test
 
+      - name: Test write-lock feature
+        run: cargo test --features write-lock
+
   lints:
     name: Lints
     runs-on: ubuntu-latest