This patch continues work started
with the previous patch.
It resolves message attribute.
Message attribute is a special attribute
in the event that may contain
references to other attributes in the
event. When message is resolved the
references are replaced with actual
values of the referenced attributes.

Started working on the async processing
and realised that I need to have a good
copy of the event with all the fields resolved
so this patch has some foundation for the async
functions (module elapi_async.c) but they
are mostly stubbed out.
The actual code will be added down the road.
Instead the patch focuses on the code
introduced in elapi_resolve.c module
and the use of the functions from it.
It also adds the implementation of the
high level calls that initialize ELAPI
with the external callbacks to be used
during async processing (elapi_log.c).