defrag: mark bctls for defrag based on number of records and size

In case when all bases are closed on blob_size limit and records_in_blob
is set way too high then blob may will try to defragment bases on each
defragmentation run.

As workaround one would probably want to lower records_in_blob to some
appropriate value.

Bug first appeared in v0.21.6
Merge pull request #47 from SaveTheRbtz/master

defrag: mark bctls for defrag based on number of records and size