Improve migration performance

Add new users to the default users group in batches of 100. The
biggest overhead of migration is in calculating the modlist when
managing the default user's group and applying the changes. A
significant amount of time can be saved by not doing this on every
add operation.

Some other minor improvements include:

Add a negative cache for groups not found in the remote LDAP server.
Replace call to user_mod with a direct LDAP update.
Catch some occurances of LimitError and handle more gracefully.

I also added some debug logging to report on migration status and
performance.

https://fedorahosted.org/freeipa/ticket/3386