Rename RelatesTo for reply messages

The bug fix for the race condition on handler execution used the new
strand interface. While it is possible to use bind_executor on every
call, that is error prone and not the suggested usage anymore.

memory leaks in the implementation.

I think I finally worked out the issues w/ linking against the
nlohmann JSON library.  I have updated the build and the various
CI scripts.

This should address comments in Nheko-Reborn#10

Makes the life of packagers and everybody who wants to build this
easier.

Closes #25.

Allowing to build against a system installed copy.