coalescing plugin handler is not optional any more

[biddisco]

If you compile a simple test app without building hpx-plugins first (only linking against libhpx and libhpx_iostreams for example), then the application will fail at runtime with an exception

{what}: attempt to create message handler plugin instance of invalid/unknown type: \
    coalescing_message_handler: HPX(bad_plugin_type)

It seems that the attempt to load the plugin should fail gracefully, but instead triggers an abort.

[biddisco]

After doing some more experiments, it appears to be a problem with release builds that come from external applications built against the hpx tree. I'm a bit confused, but I can delete the parcel_coalescing plugin from the build tree and debug works fine, but release it broken, whether it is present or not.
I will continue to experiment.

[hkaiser]

@biddisco I have it reproduced locally, even in a debug build. Will investgate...

[zao]

On this subject, last I tried building HPX the parcel plugins were not in the dependency chain of make tests, which led to an 86% test failure rate. Did that get resolved and might the lack of a suitable dependency be part of this?