peer: implement the ping/pong workflow
peer: implement the ping/pong workflow

This commit refactors the peer struct slightly in order to implement
the new ping/pong workflow added in a prior commit. Pings are currently
sent every 30 seconds unconditionally.

aakselrod/lnd,AndrewSamokhvalov/lnd,LightningNetwork/lnd,ddustin/lnd,LightningNetwork/lnd,ddustin/lnd,bryanvu/lnd,bryanvu/lnd,aakselrod/lnd,AndrewSamokhvalov/lnd