[bitshares/bitshares-core] Fix issue #125: sign block with old key if signing key updated in same block (#1203)

abitmore commented on this pull request.

> if( !(skip & skip_witness_signature) )
— FC_ASSERT( witness_obj.signing_key == block_signing_private_key.get_public_key() );
+ {
+ auto signing_key = find_witness_key_from_cache( witness_id );

It’s cheaper to check here than checking after called `_pending_tx_session.reset();`.