Add push operation to directory sync targets

[AlfredSK]

If 'Friendica node X' is using global directory server A and that directory server has enabled directory sync target B the users of 'Friendica node X' should also appear on directory server B.

They don't appear and also 'Friendica node X' isn't listed on the server list of directory server B. I think this happens because the sync is 'pull only'. This leads to incomplete directories and the local directory admin can't do anything about it.

Example:
I'm running a Friendica node on venera.social (registration policy: open). The global directory is dir.libranet.de. dir.libranet.de has three sync targets. One sync target is the official directory dir.friendica.social.
The users of venera.social are listed on dir.libranet.de but not on dir.friendica.social. And venera.social is not on the server list of dir.friendica.social.

We should change that behavior to get consistent directory listings.

Directory 2.2.1
Friendica 2019.03

[MrPetovan]

The sync directory target is currently pull only, there's no push operation yet.