"ALLOC_DEBUG" option to support debugging memory issues

Option converts memory pool and event allocation
scheme to use distinct allocations per call to
tw_calloc. Should allow detection of memory bugs
in application by tools such as valgrind.
Meant only for debugging - should not be used in
performance-sensitive runs.
Merge pull request #2 from JohnPJenkins/master

"ALLOC_DEBUG" option to support debugging memory issues