[Option 2] Fix race between handle_replica_event and start_pid_remotely

[d-b]

There appears to be a race condition leading the Swarm.Tracker process to die. When this happens the monitors that were acquired by Swarm.Tracker are lost and the process :down events will fail to be captured.

The race occurs when handle_replica_event attempts to find the remote process in the local registry. If it is not found, but is then registered in the mean time by start_pid_remotely an error will occur when handle_replica_event tries to register the process itself

swarm/lib/swarm/tracker/tracker.ex

Lines 1004 to 1008 in 7dd08d6

	:undefined ->
	ref = Process.monitor(pid)
	lclock = Clock.join(clock, rclock)
	Registry.new!(entry(name: name, pid: pid, ref: ref, meta: meta, clock: lclock))
	:keep_state_and_data

This solution removes the call to add_registration from start_pid_remotely to remove the potential race condition. This reintroduces the issue bitwalker#45 fixed by bitwalker#46. Option 1 may be preferable.

[Corvillus]

As we decided on option 1 I'm removing myself from the reviewers on this PR. If this comes back, feel free to re-add me.