mtmd: add mtmd_log_set (breaking change)

[ngxson]

Small breaking change:

• mtmd_context_params.verbosity is removed
  • by default, everything will be logged to stderr
  • provide your custom log function via mtmd_log_set()
• If you don't care about logging, simply delete verbosity from your mtmd_context_params

// this will call mtmd_log_set() internally
// logs will be redirect to either stderr or files, based on common_log config
mtmd_helper_log_set(common_log_default_callback, nullptr);