rewind: guard against rewinding to different predecessors at the same time

One case that is a bit weird, but should also be somehow handled, is when user
asks to rewind to many predecessor sets, e.g. with 'allpredecessors(X)' or even
with 'obsolete()'. Lets make rewind complain and abort by default.

A new experimental config option is introduced to allow users to force such
rewinds in case we have missed some valid use cases.

It's also been agreed to not use --exact or --as-divergence flags to ignore
this predecessors check.
8 jobs for topic/default/rewind-dry-run in 6 minutes and 22 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #119293
checks-py2

00:02:58

passed #119294
checks-py3

00:02:52

passed #119299
doc

00:00:52

manual #119292
allowed to fail manual
pytype
passed #119295
tests-py2-cext

00:06:21

passed #119296
tests-py2-pure

00:04:07

passed #119297
tests-py3-cext

00:06:17

passed #119298
tests-py3-pure

00:03:59