This commit introduces a new `:handover_on_rejoin` channel option that
can be used to do a custom handover when rejoining channels.

Relates to phoenixframework/phoenix_live_view#3357.

This commit relies on a yet to be finalized feature in Phoenix Channels
to perform a custom handover between channel rejoins.

It changes the LV code to not leave the old channel before rejoining and
also instructs Phoenix to not kill the old process before starting the
new channel process. After the new channel is joined, the old one is
killed. Any `assign_new` calls in the LV mount will try to fetch assigns
from the old LV.

This is a backwards compatible optimization, so if a version of Phoenix
is used that does not support handover, it just falls back to calling the
function supplied to assign_new as usual.

Closes #3357.

This commit relies on a yet to be finalized feature in Phoenix Channels
to perform a custom handover between channel rejoins.

It changes the LV code to not leave the old channel before rejoining and
also instructs Phoenix to not kill the old process before starting the
new channel process. After the new channel is joined, the old one is
killed. Any `assign_new` calls in the LV mount will try to fetch assigns
from the old LV.

This is a backwards compatible optimization, so if a version of Phoenix
is used that does not support handover, it just falls back to calling the
function supplied to assign_new as usual.

Closes #3357.

This commit relies on a yet to be finalized feature in Phoenix Channels
to perform a custom handover between channel rejoins.

It changes the LV code to not leave the old channel before rejoining and
also instructs Phoenix to not kill the old process before starting the
new channel process. After the new channel is joined, the old one is
killed. Any `assign_new` calls in the LV mount will try to fetch assigns
from the old LV.

This is a backwards compatible optimization, so if a version of Phoenix
is used that does not support handover, it just falls back to calling the
function supplied to assign_new as usual.

Closes #3357.

This commit relies on a yet to be finalized feature in Phoenix Channels
to perform a custom handover between channel rejoins.

It changes the LV code to not leave the old channel before rejoining and
also instructs Phoenix to not kill the old process before starting the
new channel process. After the new channel is joined, the old one is
killed. Any `assign_new` calls in the LV mount will try to fetch assigns
from the old LV.

This is a backwards compatible optimization, so if a version of Phoenix
is used that does not support handover, it just falls back to calling the
function supplied to assign_new as usual.

Closes #3357.

This commit relies on a yet to be finalized feature in Phoenix Channels
to perform a custom handover between channel rejoins.

It changes the LV code to not leave the old channel before rejoining and
also instructs Phoenix to not kill the old process before starting the
new channel process. After the new channel is joined, the old one is
killed. Any `assign_new` calls in the LV mount will try to fetch assigns
from the old LV.

This is a backwards compatible optimization, so if a version of Phoenix
is used that does not support handover, it just falls back to calling the
function supplied to assign_new as usual.

Closes #3357.