Fix/GC: Do not recursively descend all internal pins

InternalPins() is a pinset composed by:

- Recursive pins CIDs
- Direct pins CIDs
- The empty node CID
- A root CID pointing to all above (and any of the subbuckets that may have been created)

It is only set during Flush/Load operations for the pinner.

Thus recursively exploring internal pins in order to decide which CIDs are safe
from GC only re-explores the recursive DAGs and should not be necessary.

Mind that, previously, the CidSet will correctly prune any already explored branches so
it did not have pernicious effects. But now it does.

License: MIT
Signed-off-by: Hector Sanjuan <code@hector.link>