Memtx checkpointing proceeds as follows: first we open iterators over
primary indexes of all spaces and save them to a list, then we start
a thread that uses the iterators to dump space contents to a snap file.
To avoid accessing a freed tuple, we put the small allocator to the
delayed free mode. However, this doesn't prevent an index from being
dropped so we also take the schema lock to lock out any DDL operation
that can potentially destroy a space or an index. Note, vinyl doesn't
need this lock, because it implements index reference counting under
the hood.

Actually, we don't really need to take a lock - instead we can simply
postpone index destruction until checkpointing is complete, similarly
to how we postpone destruction of individual tuples. We even have all
the infrastructure for this - it's delayed garbage collection. So this
patch tweaks it a bit to delay the actual index destruction to be done
after checkpointing is complete.

This is a step forward towards removal of the schema lock, which stands
in the way of transactional DDL.