lib: directly traverse postlists in _n_message_delete

This is intended to fix the slow behaviour of "notmuch new" (and possibly
"notmuch reindex") when large numbers of files are deleted.

The underlying issue [1] seems to be the Xapian glass backend spending
a large amount of time in db.has_positions when running queries with
large-ish amounts of unflushed changes.

This commit removes two uses of Xapian queries [2], and replaces them with
an approximation of what Xapian would do after optimizing the
queries. This avoids the calls to has_positions (which are in any case
un-needed because we are only using boolean terms here).

[1] Thanks to "andres" on IRC for narrowing down the performance
bottleneck.

[2] Thanks to Olly Betts of Xapian fame for talking me a through a fix
that does not require people to update Xapian.

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