This is done to avoid situation, when two or more
fibers yield inside their try/catch blocks and
throws an exceptions. Seems like exception unwinder
stores some global state while being inside
a catch block.

This could lead to incorrect exception processing
and server crash.

The patch should use struct session, which is non-existent
in 1.5, so at least make session_storage_destroy a bit faster.

This patch implements show index and shows memory waste of the
slab allocator.

Fixes gh-153.

-related distributions due to libbfd crash