lei_xsearch: cross-(inbox|extindex) search

While a single extindex combines multiple inboxes into a single
search index, extindex still requires up-front indexing on items
which can be searched.  XSearch has no on-disk footprint itself
and uses Xapian DBs of existing publicinbox and extindex
("extinbox") exclusively.

XSearch still suffers from the multi-shard Xapian scalability
problems which led to the creation of extindex, but I expect the
number of shards to remain relatively low.

I envision users hosting public-inbox instances on their
workstations will only have two extindex combined by this, one
read-only extindex for serving public archives, and one
read-write extindex managed by LeiStore for private mail.

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