Provide an option to skip navigation menu creation in the create domain API call, since it seems prone to deadlock

I have found the _civicrm_load_navigation() call within the civicrm_api3_multisite_domain_create() API call prone to deadlock, causing the API call to hang and ultimately fail.

As a workaround, I've added optional skip_nav parameter. When true (or truthy), the _civicrm_load_navigation() call is skipped.

With this in place, I can create a new domain without error by:

1. Call civicrm_api3_multisite_domain_create() including $params['skip_nav'] = 1. The API call succeeds.
2. Call civicrm_api3_multisite_domain_create() with the same parameters as step 1, except omitting $params['skip_nav'] (or with $params['skip_nav'] = 0). Since the domain already exists, it is not recreated. And on this second API call, the _civicrm_load_navigation() does not ever seem to deadlock, so the navigation records for the new domain are successfully created.

I've combined this with the suggested fix to #7 (closed) and will submit a merge request for this.