Merge tag '0.28' into develop

CppCon-Release:
- fixed bugs found during preparation for the talk
- added examples for the talk
- aggregator functions cannot be used in where() anymore (thanks to Arthur O'Dwyer for finding that in the open content session)
- fixed serialization of dynamic query elements (the separator was missing, thanks to Paul Körbitz for finding that)
- several minor cleanups
Fixed bug in in() serialization

in(select(...)) did have an extra set of braces, which lead to different behaviour.
Thanks to Thomas Marsh for finding the bug
Merge branch 'release/0.29'