API: Add generator non-continuation data

API: Add generator non-continuation data

Some generators (e.g. generator=recentchanges) in their default mode of
operation are particularly prone to not generating the same result set
on subsequent requests due to intervening activity on the wiki adding
new entries to the start of the list. We can mitigate this effect by
allowing such generators to provide "non-continuation" data to be used
if the generator isn't being continued.

ApiQueryRecentChanges and ApiQueryAllRevisions are updated to set this
new property. Other generators can easily be updated in the same way as
needed.

There isn't anything we can do about generators prone to having entries
added at random positions in the list rather than the beginning,
unfortunately.

Bug: T146176
Change-Id: I8308d6aa2c89fd2a85e74c7dd8a0a2a9ec927490