Disk space report

Report available disk space on nodes and calculate cluster-wide available space in `garage stats` (fix #479)

Cargo.lock

@@ -1230,6 +1230,7 @@ dependencies = [
  "serde",
  "serde_bytes",
  "serde_json",
+ "systemstat",
  "tokio",
  "tokio-stream",
  "tracing",
@@ -3567,6 +3568,20 @@ dependencies = [
  "unicode-xid",
 ]
 
+[[package]]
+name = "systemstat"
+version = "0.2.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a24aec24a9312c83999a28e3ef9db7e2afd5c64bf47725b758cdc1cafd5b0bd2"
+dependencies = [
+ "bytesize",
+ "lazy_static",
+ "libc",
+ "nom",
+ "time 0.3.9",
+ "winapi",
+]
+
 [[package]]
 name = "tempfile"
 version = "3.3.0"