btrfs: Mount with `flushoncommit` flag

Not an expert on this, but my understanding is that without this flag,
outages will result in a state that despite being consistent, most
applications are not mature enough to handle. Namely, we ran benchmarks
that reproduced appearance of zero-length files upon sudden poweroffs.

Databases should be fine since they know well about the guarantees the
filesystem must provide, but not applications are databases. So let's
play safe about this.

See:
- https://thunk.org/tytso/blog/2009/03/12/delayed-allocation-and-the-zero-length-file-problem/
- https://github.com/Zygo/bees/issues/68#issuecomment-403262059

declarative.py

@@ -40,7 +40,13 @@ def be_mounted(dev, mountpoint):
         # noinspection PyUnreachableCode
         be_unmounted(mountpoint)
 
-    run(f"mount {dev} {mountpoint}")
+    fs = current_fs(dev)
+    if fs == "ext4":
+        run(f"mount {dev} {mountpoint}")
+    elif fs == "btrfs":
+        run(f"mount -o flushoncommit {dev} {mountpoint}")
+    else:
+        raise Exception(f"Unsupported fs type: {fs}")
 
 
 def be_unmounted(path):