Adds allowed-peers Flag

[otherview]

Description

Adds a new flag that limits the connections of a node.
This features help and allows for a few features like bootstrapping to be limited to a single node (good for metrics), DMZ's and traffic-shapping in general is now easier.

Flags accept a list of enodes :

--allowed-peers enode://797fdd968592ca3b59a143f1aa2f152913499d4bb469f2bd5b62dfb1257707b4cb0686563fe144ee2088b1cc4f174bd72df51dbeb7ec1c5b6a8d8599c756f38b@107.150.112.22:55555 enode://3eae6740af6180bb015309f7a07ff7405d6f1f9f1e5a9f2fabbd36b0c00b862521e63ff23573ffdb9035f2237c26513cb9f02454f9ada993e60b99ffc187bb54@107.150.112.21:55555

Fixes # (issue)

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Log testing, making sure that no other peer had connected

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 5.26316% with 36 lines in your changes are missing coverage. Please review.

Project coverage is 61.35%. Comparing base (2441238) to head (12816f0).

Files	Patch %	Lines
cmd/thor/utils.go	0.00%	34 Missing ⚠️
cmd/thor/main.go	0.00%	1 Missing ⚠️
p2psrv/server.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #688      +/-   ##
==========================================
+ Coverage   61.24%   61.35%   +0.11%     
==========================================
  Files         194      194              
  Lines       18207    18221      +14     
==========================================
+ Hits        11150    11179      +29     
+ Misses       5975     5960      -15     
  Partials     1082     1082              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[claytonneal]

heya, do we need up update any README or docs.vechain.org for this?

[libotony]

We already have a --bootnode flag that does part of the feature but does not limit other connections. I would consider adding a new flag --no-discovery that disables the discovery feature.

As the --bootnode flag indicates bootstrapping/discovery node, might not be ideal to indicate that it's the nodes that used to be connecting via peer-to-peer network, a new flag can be added for the designated feature.

And for the --no-discovery option, I think it's better to be a hidden flag, as it won't be a general usage and shouldn't be used in general cases.

[otherview]

Myself and @libotony did a pair-prog on this, due to flag behaviours we ended up agreeing on allow-peers-only as a flag that only allows connectivity to certain peers.

The TLDR is that --no-discovery + --bootnodes in certain cases can still allow connections to other nodes.

[darrenvechain]

Non-blocking thoughts around the flag, but looks good

[paologalligit]

Myself and @libotony did a pair-prog on this, due to flag behaviours we ended up agreeing on allow-peers-only as a flag that only allows connectivity to certain peers.

The TLDR is that --no-discovery + --bootnodes in certain cases can still allow connections to other nodes.

About the last line, is it because this is only an outgoing connection restricted list and so it doesn't imply that another node could start an incoming connection with the node (even if not in the allow-peers flag)?