ai_helper.get_closest_location(): fix bug with tags in SLF

The way how the Standard Location Filter was dealt with only worked
with keys, not tags.
Messenger MAI: fix bug with escort units blocking the messenger

When there were lots of escort units, they previously quite frequently
blocked the messenger’s way, preventing him from making progress even
when it was possible.  Escort units now specifically move out of the
way if they do not need to attack enemy units in the messenger’s way.
Messenger MAI: allow several messengers to be controlled by AI

For two of the CAs this mostly just means reading the [filter] tag and
picking one of the messengers.  However, for the CA moving the escort
units, it means a complete rewrite, as the AI now needs to figure out
which escort unit should be moved toward which messenger.
Messenger MAI: add optional invert_order= key

In the default configuration, the AI moves the messenger which has
advanced the furthest through the waypoints first.  If this key is set,
the rearmost messenger is moved first.
MAIs: minor code simplification in AIs that take both [filter] and id=

These are the AIs that used to use Behavior Candidate Actions (BCA).
Patrol MAI: fix bug with save/load persistent data storage

WML values may not be used as keys, this can make savegames invalid.
Store information in the units now rather than in self.data. (Note:
storing in self.data would also be possible if the syntax were changed.)
Merge branch 'master' of https://github.com/wesnoth/wesnoth