Copy-by-value in sdbus++ generated objects

[Patrick Venture]

Most repos inherit from an auto-generated object.  Those objects pass
all parameters by value.  This is fine for many cases where they would
need to create a copy of it, but in the case where:

for instance:
https://github.com/openbmc/sdbusplus/blob/master/tools/sdbusplus/templates/property.mako.prototype.hpp.in#L69

The values match it's wasteful to copy.  It's also possible the cpp I
learned way back is different in how this behavior works, but I wanted
to check.  Because of the prevalence of this code, if there are lots
of updates that don't actually change the value, it could optimize
quite a bit at run-time.

It would require a lot of simultaneous fixes (or two different objects
generated, and transitioning everything).

Thoughts?

Patrick