Fix light connection issues

[svyatonik]

There are 3 issues currently (each one takes its own commit):

• the main issue that causes (almost) instant disconnect is that consensus gossip messages are sent to light nodes && light nodes do not have gossip listeners registered. I've fixed that by ignoring light nodes in ConsensusGossip - think that is the correct way;
• the second issue is a typo in LightCallExecutor::version - instead of "version" it should call "Core_version". Currently aura verifier calls this - that should be fixed by Light GRANDPA import handler #1669 ;
• the third one is the most complex, linked to the peer reputation system. The initial node reputation is zero. Normally, when full nodes are connected to each other, they first interchanging with GRANDPA messages => their reputation is increased. But because of (first issue) above, we do not send gossip messages to light nodes. And when some block is announced, reputation of the peer is changed by -4 here. This leads to the situation when node has negative reputation and we disconnect it here. @tomaka I need your help here - there are several ways to fix this, but I'm not sure what's better. Also - comment to BLOCK_ANNOUNCE_REPUTATION_CHANGE states that 'Since this has a small cost, we decrease the reputation of the node, and will increase it back later if the import is successful.' - I haven't found that. Could you, please, point me to that code? Probably it isn't executed on light, because I have seen on-announce-disconnects even after peer has provided us with several valid blocks.

Adding suggested reviewers to the PR (skipping Rob :) ).
Cc @andresilva and @niklasad1 (I'm not sure, but the second issue could be the issue you have found today as well).

[tomaka]

@tomaka I need your help here - there are several ways to fix this, but I'm not sure what's better.

Good point, I think that's missing actually.
#2440 would fix the instant disconnect, but the reputation added back on import is indeed missing I think.

[svyatonik]

Decided to remove penalty from on_announce_block at all, after chatting with Pierre. The reputation will slowly decay towards zero after #2440 anyway.