rpc: decrease websocket ping interval

[ucwong]

This is to cater for more node providers.

[codecov]

Codecov Report

Merging #1370 (911ed09) into master (5a26be7) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1370      +/-   ##
==========================================
- Coverage   51.65%   51.61%   -0.05%     
==========================================
  Files         461      461              
  Lines       58048    58048              
==========================================
- Hits        29987    29960      -27     
- Misses      25792    25813      +21     
- Partials     2269     2275       +6     

Impacted Files	Coverage Δ
rpc/websocket.go	75.48% <ø> (ø)
trie/sync_bloom.go	58.33% <0.00%> (-5.96%)	⬇️
whisper/whisperv6/whisper.go	54.01% <0.00%> (-1.20%)	⬇️
whisper/whisperv6/message.go	69.94% <0.00%> (-1.16%)	⬇️
trie/proof.go	75.45% <0.00%> (-0.91%)	⬇️
p2p/simulations/mocker.go	33.63% <0.00%> (-0.91%)	⬇️
core/txpool/txpool.go	72.82% <0.00%> (-0.73%)	⬇️
p2p/simulations/network.go	59.85% <0.00%> (-0.72%)	⬇️
ctxc/filters/api.go	50.66% <0.00%> (-0.67%)	⬇️
p2p/simulations/http.go	69.56% <0.00%> (-0.55%)	⬇️
... and 4 more