Extend duplicate shred checks for overlapping erasure sets.

[AshwinSekar]

Problem

Currently we only detect duplicate coding shreds that are part of the same fec set and have incompatible erasure sets.

Proposed Solution

From behzad:
Any 2 shreds with different but overlapping erasure sets can be considered duplicate.
In particular doesn't even need to have the same fec_set_index.

Technically the overlapping erasure sets might result in non-duplicate shreds, but a leader shouldn't generate overlapping erasure sets unless malicious or buggy.

We should extend the check in blockstore and relax the constraint in gossip to handle proofs for these kind of conflicts.