Revise epix_merge to always fit in last-version-carried-forward framework, merge additional metadata

[brookslogan]

as_of and other planned epi_archive operations assume we formed an archive using last-version-carry-forwardable data. Using epix_merge without all=TRUE and locf=TRUE breaks this.

locf=TRUE can produce a result that gives inconsistent as_of results with the input archives if the user has NAs in the input archive non-key columns. We want to locf only for rows that didn't exist before, at least when interpolating versions, and always want to locf when interpolating versions. However, there are other cases, such as when one archive has more up-to-date version data than the other, where we wonder about what, if any, extrapolation we should be doing; locf should be replaced with an observed_versions_end_conflict arg that allows various options there (e.g., to raise an error if one input isn't as fresh, to insert an all-NA version immediately after the last version in the less fresh input, to extrapolate with last-observation-carried-forward, or to truncate versions from the fresher input).

Additionally, epix_merge should appropriately check input metadata and set result metadata: the DT key, geo and time types, clobberable and observed versions, etc.

[brookslogan]

Closed by #101.