Discv5 Improvements

[kdeme]

Improvements / bugfixes / features that are most important for nbc, in order of priority:

• Make sure FindNode works properly: Make FindNode according to spec nim-eth#205
• Passing ENR custom fields: needed for https://github.com/ethereum/eth2.0-specs/blob/v0.11.1/specs/phase0/p2p-interface.md#eth2-field , see also [Apr20] 0.11.1 networking updates #832.
• Filtering discovered nodes based on ENR custom fields: needed for https://github.com/ethereum/eth2.0-specs/blob/v0.11.1/specs/phase0/p2p-interface.md#eth2-field , see also [Apr20] 0.11.1 networking updates #832. Without this one we will still possible try to dial with a node that is not useful.
• Fix error handling: Improve/fix Error handling in discovery v5 nim-eth#185
• Checks on AuthResponse: Add checks on AuthResponse nim-eth#204
• Liveness functionality: Liveness verified functionality nim-eth#183
• Better (re)validation: Improve (re)validation of nodes nim-eth#206
• Allow to update ENR: Allow for updating ENR and the enr-seq number nim-eth#208

Tests:

• More tests, e.g.: More tests for discoveryv5 nim-eth#181

[kdeme]

Decided to change focus more on status-im/nim-eth#185 and work on #183 and #206 at later time

[kdeme]

So, this item can be moved to next May milestones.
Initial focus was shifted from adding features to first work on the error handling. Some ground work was done there (status-im/nim-eth#231) but more is to be done.

[kdeme]

I'd close this instead of move as the last item is handled here: status-im/nim-eth#266
And while not all tests mentioned in that test item are handled, several other (not mentioned are).
So probably better to create a new item with which discovery items to handle.