[bug]: Ping time is not accurate

[alexbosworth]

Background

When calling ListPeers API, the list peer response returns an inaccurate ping time for a peer

Your environment

• lnd 0.18.0-beta.rc1

Steps to reproduce

• Add a peer

Expected behavior

• The accurate peer ping time should be reflected

Actual behavior

• Peer ping time is returned as '-1' which is not a possible time according to the rules of relativity

[bitromortac]

This is a default value for situations where a ping time wasn't measured:

https://github.com/lightningnetwork/lnd/commit/99226e37efc7c0b72f53c66d0e35a7799d1f6ab0#diff-1b1c0e1c6693b8d2fba799853f952e05a33461e8127c1b876a1d95a8f7da0c79R247-R256

Previously this would have been 0, I think.