broken p2p communication in protocol-feature-foundations

[taokayan]

in branch protocol-feature-foundations, if a block is received from a peer, it will likely be broken at the time it is broadcast to another peer, resulting in the following exception:

warn  2019-02-28T09:41:19.503 thread-0  controller.cpp:1489           push_block           ] 0 exception: unspecified
unable to reconstruct public key from signature
    {}
    thread-1  elliptic_secp256k1.cpp:160 public_key
error 2019-02-28T09:41:19.503 thread-0  producer_plugin.cpp:320       on_incoming_block    ] 0 exception: unspecified
unable to reconstruct public key from signature
    {}
    thread-1  elliptic_secp256k1.cpp:160 public_key
rethrow
    {}
    thread-0  controller.cpp:1489 push_block

This is unrelated to the speculative or irreversible mode, and can be reproduce by the following setup:
[BP node] -> [ peer 1 ] -> [ peer 2 ]
and the exception message is on peer2.

However if peer 1 is running the old code (release/1.6.x), this issue will not exist.

[taokayan]

#6736

[arhag]

Resolved by #6856.