userdb: Use protocol buffers instead of our custom format

Protocol buffers are a more portable, practical and safe format for the user
database.

test/t-01-simple_local/run.sh

@@ -6,6 +6,7 @@ set -e
 init
 
 generate_certs_for testserver
+add_user testserver user secretpassword
 
 mkdir -p .logs
 chasquid -v=2 --log_dir=.logs --config_dir=config &