Add logging to export the list of licensed Confluence users

Currently it is difficult to find out which users are taking up a license. It would be great to be able to see this. A quick way to do it would be to provide a log level that displays the list of users.

After this is implemented users will need to follow the following steps to see the list of licensed users:

1. Go to the "Logging and profiling" administration screen
2. Add an entry for "com.atlassian.confluence.user.DefaultUserAccessor" with level "DEBUG"
3. Go to the "License Details" administration screen
4. Tail your Confluence log file
5. Hit the "Refresh" button under the "Licensed users" section
6. Look for a line like this in your log file

2019-09-17 10:36:34,271 DEBUG [read-only-transaction:thread-1] [atlassian.confluence.user.DefaultUserAccessor] getUserNamesWithConfluenceAccess Found n licensed users: [user1, user2, ...]