This function migrates more poll functions (including keepalive) to
poll.go to keep it somehow in the same file.
In addition it makes changes to improve the stability and ensure nodes
get the appropriate updates from the headscale control and are not left
in an inconsistent state.
Two new additions is:
omitpeers=true will now trigger an update if the clients are not already up
to date
keepalive has been extended with a timer that will check every 120s if
all nodes are up to date.
