p2p: move ping handling into pingLoop goroutine

[ucwong]

Moving the response sending there allows tracking all peer goroutines in the peer WaitGroup.

[codecov]

Codecov Report

Merging #1670 (92fefb2) into master (9b6ca64) will decrease coverage by 0.03%.
Report is 2 commits behind head on master.
The diff coverage is 53.84%.

❗ Current head 92fefb2 differs from pull request most recent head daa21a1. Consider uploading reports for the commit daa21a1 to get more accurate results

@@            Coverage Diff             @@
##           master    #1670      +/-   ##
==========================================
- Coverage   52.60%   52.58%   -0.03%     
==========================================
  Files         481      481              
  Lines       60233    60238       +5     
==========================================
- Hits        31686    31675      -11     
- Misses      26132    26155      +23     
+ Partials     2415     2408       -7     

Files Changed	Coverage Δ
core/block_validator.go	41.17% <0.00%> (ø)
ctxc/config.go	40.00% <ø> (ø)
miner/miner.go	0.00% <ø> (ø)
miner/worker.go	0.00% <0.00%> (ø)
p2p/peer.go	74.28% <85.71%> (+0.11%)	⬆️
consensus/cuckoo/consensus.go	8.69% <100.00%> (ø)

... and 13 files with indirect coverage changes