join: Refactor clone_only case to simplify code

join: Refactor clone_only case to simplify code

Currently for DC clones, we create a regular DCJoinContext, se a
'clone_only' flag, and then make lots of special checks for this flag
throughout the code. Instead, we can use inheritance to create a
DCCloneContext sub-class, and put the specialization there.

This means we can remove all the 'clone_only' checks from the code. The
only 2 methods that really differ are do_join() and join_finalize(), and
these don't share much code at all. (To avoid duplication, I split the
first part of do_join() into a new build_nc_lists() function, but this
is a pretty trivial code move).

We still pass the clone_only flag into the __init__() as there's still
one case where we want to avoid doing work in the case of the clone.
For clarity, I'll refactor this in a subsequent patch.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>