v4 of index multiple files per msg-id, add reindex command

[David Bremner]

This is mainly a rebase of

     id:20170604123235.24466-1-david at tethera.net

against current master (roughly 0.25~rc1)

There is a bit of movement between patches to make sure tests pass
after every patch, but the resulting interdiff is basically trivial

diff --git a/lib/database-private.h b/lib/database-private.h
index 504618b9..5555554b 100644
--- a/lib/database-private.h
+++ b/lib/database-private.h
@@ -252,8 +252,4 @@ _notmuch_database_find_doc_ids (notmuch_database_t *notmuch,
                                const char *value,
                                Xapian::PostingIterator *begin,
                                Xapian::PostingIterator *end);
-
-
-#pragma GCC visibility pop
-

AFAIK, that has to do with Jani's changes to visibility in the library.

Unless I hear otherwise, I'll probably merge this series to master
shortly after the 0.25 release.