BFT Block Puller: make sure invalid headers don't go beyond config-index

[tock-ibm]

Let's assume the last config block is Bm, i.e. config-index=m.

If the block target orderer stops right before the next config block Bn, and Bn makes future blocks invalid if verified with Bm, the headers receivers should not go beyond block n.
Since invalid headers are ignored, if they do go ahead, this may cause the censorship event to go undetected.

A simplification of the rule above is not to allow header receivers to go beyond the config index of the last block. This way, a censorship attempt of a config block is always detected.

[tock-ibm]

This issue has been addressed by having independent verifiers for header receivers and the block receiver.