Problem: We have code which is only compiled if the macro EXTRA_DEBUG
is defined (definable from cmake using -DEXTRA_DEBUG=1). Some of this
code currently does not compile.

Solution: Fix EXTRA_DEBUG-code which:
 - used function my_message_stderr() as a varargs function.
 - assumed Table_cache was a home-grown hashtable rather than an unordered_map
 - assumed entry->file->extra(HA_EXTRA_IS_ATTACHED_CHILDREN is always false
   for free tables (runtime error).

Change-Id: Ie91e8b2058d465f7a3a9d10c3ee00549b23bbe17