containers are not required to allocate, e.g. small string optimization
for now this is hypothetical since we cannot use std::basic_string with
at least gcc 7 since the basic_string there still uses the deprecated
rebind allocator interface (which pmr does not implement)
the patch seems big but most of it is just propagating the new notion of
constness of the factory - since it keeps track of created messages with
the internal allocator it no longer is const
