core: fix memory leak in core_memory_pool for large segments

Large segments (not fulfilled by memory blocks) are never freed
when the flag FLAG_ENABLE_TRACKING is cleared.

This bug was introduced in commit 153eea9 ("thorium_node: use
message type to select pool to free buffer").

This patch adds a fallback code path for large segments when
FLAG_ENABLE_TRACKING is cleared.

Link: https://github.com/GeneAssembly/biosal/issues/788
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>