blob: prepare for switch to unsorted index iteration

This also should fix crashes in case of multiple iterators used, because
previously any subsequent iterator could remap bctl->data used by
another iterator.
Also we protecting eblob_base_setup_data() calls agains each other via
lock.
blob: check that bctl->data is still valid on iteration
blob: do not disable defragmentation while iterating

Now that we have proper locking on iteration path it should be safe.