Add search_after option to search api for pagination

[martijnvg]

Adding search_after option to the search api will allow better deep pagination support for search request that don't use the scroll api.

The search_after option should be used to hold the last seen sort value for each search request being executed with the same query (just like updating the from option). Although in this case we don't keep open a consistent view of all the shards, like we do with scroll search, it is a good alternative for deep pagination for normal search requests that use from option. (actually the only one that doesn't cause jvm heap issues without maintaining open search contexts during subsequent scroll requests)

[clintongormley]

+1

[clintongormley]

Duplicate of #8192