Selective database archiving

@davisp:
> When you say selective, do you mean selectively archiving parts of a database? And is this internal or external? And/or does this mean make it read-only or read-not-at-all and/or I dunno.

@janl:
> I think the selective option would be again something like “after 90 days, put docs in archive db (on cheaper storage)”.

@wohali:
This is basically native support for things that we currently have to do using rolling databases or similar. It could go along with the partitioning support that is currently being worked on by @garrensmith and @rnewson and proposed by @mikerhodes .

Start by resolving the unanswered scope in issue #1508: define whether selective archiving means moving documents after a retention period, making them read-only, or excluding them from reads. Review the relationship to rolling databases and the partitioning work mentioned in the issue; done requires an agreed design and implementation boundaries.

Written by the indexing model from the issue text.