node: Populate refs db on startup

If the refs database is empty, populate it on startup.
Also includes a script to clear it.

scripts/clear-refs-db.sh

@@ -0,0 +1,18 @@
+#!/bin/sh
+set -e
+
+DB="$(rad path)/node/node.db"
+
+if command -v sqlite3 >/dev/null 2>&1; then
+  if [ -f "$DB" ]; then
+    echo -n "Clearing 'refs' table from $DB.. "
+    sqlite3 $DB "DELETE FROM refs;"
+    echo "done."
+  else
+    echo "fatal: database file does not exist"
+    exit 1
+  fi
+else
+  echo "fatal: sqlite3 is not installed"
+  exit 1
+fi