log: remove "boost/asio.hpp" from Log.h

log: remove "boost/asio.hpp" from Log.h

graylog uses boost/asio.hpp which introduces the link-time dependency on
libboost_system to the compilation units which includes Log.h and
LogClient.h. so it appears that perfglue/heap_profiler.cc is referencing
libboost_system.so, and fails the cmake build of all tests which links
against tcmalloc.

in this change, we:

* remove unnecessary #includes from Graylog.h
* forward declare Graylog class, so that "Graylog.h" is not included in
  any header files to avoid the link-time dependency pollution

Signed-off-by: Kefu Chai <kchai@redhat.com>