box: make cfg value comparison more thorough

Use table.equals for comparing table-valued configuration parameters
instead of ipairs. Needed to trigger reconfiguraiton if the new URI
differs from the old one only by parameters, e.g.:

  box.cfg{listen = {uri = 3301, params = {transport = 'plain'}}}
  box.cfg{listen = {uri = 3301, params = {transport = 'ssl'}}}

No test is added, because currently there's no URI parameters handled
by box. The next commit adds the 'transport' parameter and a test that
wouldn't pass without this commit.