Support prev/next 100 messages for the message viewer

[twmb]

Console supports pagination for the last 500 messages, in that the message viewer allows navigating within those 500 messages. We'd like the ability to load the next 100 messages or the previous 100 messages as well, and then continue navigating onward.

Perhaps one way to do this is to just continue loading the next batch of messages as people navigate through the pages.

[weeco]

If your search targets the "most recent 500 messages" and you scroll down (to older messages), what would you expect to happen at the end when you ask for the next page? Would it suddenly show newer messages?

Also: Because we consume across partitions the search is a bit opinionated and the results are not necessarily stable. We try to serve a balanced number of messages across all partitions (rather than just showing 500 messages from any partition). This oftentimes means some partition will e.g. yield 10 messages and others 9. We optimized for speed and it doesn't matter whether 10 or 9 messages came from partition 1 or partition 3 when both is okay and leads to an even result across all partitions.