This commit tries to address the possible raceondition that can happen
if a client closes its connection after we have fetched it from the
syncmap before sending the message.
To try to avoid introducing new dead lock conditions, all messages sent
to updateChannel has been moved into a function, which handles the
locking (instead of calling it all over the place)
The same lock is used around the delete/close function.
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.
