Allow using untilTimeLimit in EMIT FINAL statement for windowed aggregations

It will be useful to add an option to the EMIT FINAL statement (or using other options as well) in order to use the `untilTimeLimit` instead of `untilWindowCloses` (the default value now, see #1030). This will allow the rate limiting of aggregation queries (es. calculating the average values over a time period).

Start by reading the EMIT FINAL statement behavior and the prior discussion in issue #1030, focusing on how untilWindowCloses is selected for windowed aggregations. Determine where untilTimeLimit could be specified and define completion by having aggregation queries honor that limit while preserving the existing default behavior.

Written by the indexing model from the issue text.