Don't allow releases when TODO(now) or TODO(soon) or KJ_DBG are found in the code.

release.sh

@@ -2,6 +2,16 @@
 
 set -euo pipefail
 
+if (grep -r KJ_DBG c++/src | egrep -v '/debug(-test)?[.]'); then
+  echo '*** Error:  There are instances of KJ_DBG in the code.' >&2
+  exit 1
+fi
+
+if (egrep -r 'TODO\((now|soon)\)'); then
+  echo '*** Error:  There are release-blocking TODOs in the code.' >&2
+  exit 1
+fi
+
 doit() {
   echo "@@@@ $@"
   "$@"