miscsearch: a new Xapian sub-DB for extindex

This will be used to index and search Inbox objects and perhaps
individual git repositories/epochs for grokmirror manifest.js.gz
generation.  There is no sharding planned for this at the moment
since inbox count should remain low (~100K to 1M) compared to
message count.

Folding this into the existing sharded DBs could be possible;
but would likely increase query and maintenance costs, as well
as development complexity.  So we'll use a few more inodes and
FDs at runtime, instead.

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