[WIP] Using aliases to allow reindexing with atomic alias switching

Rebased #264 on current master.

Todo:
- [ ] fix tests
- [ ] fix fos:elastica:reset that doesn't truncate anymore
- [ ] think of an alternative implementation because this is bound to fos:elastica:populate. but not all people use it to index data, but they still want to make use of aliases for reindexing/mapping update without downtime