More work on `git annex export`. Made `initremote exporttree=yes` be
required to enable exporting to a special remote. Added a sqlite database
to keep track of what files have been exported. That let me fix the known
problems with exporting multiple files that have the same content. 

The same database lets `git annex get` (etc) download content from exports.
Since an export is not a key/value store, git-annex has to do more
verification of content downloaded from an export. Some types of keys,
that are not based on checksums (eg WORM and URL), 
cannot be downloaded from an export. And, git-annex will never trust
an export to retain the content of a key, since some other tree could
be exported over it at any time.

With `git annex get` working from exports, it might be nice to also support
`git annex copy --to export` for exporting specific files to them. However,
that needs information that is not currently stored in the sqlite database
until the export has already completed. One way it could work is for `git
annex export --fast treeish --to export` to put all the filenames in the
database but not export anything, and then `git annex copy --to export` (or
even `git annex sync --content` to send the contents). I don't know if this
complication is worth it.

Otherwise, the export feature is fairly close to being complete now.
Still need to make renames be handled efficiently, and add support for
exporting to more special remotes.

Today's work was supported by the NSF-funded DataLad project.
