searchidx: do not modify Xapian DB while iterating
Iterating through a list of documents while modifying them does
not seem to be supported in Xapian and it can trigger
DatabaseCorruptError exceptions. This only worked with past
datasets out of dumb luck. With the work-in-progress "v2"
public-inbox layout, this problem might become more visible
as the "thread skeleton" is partitioned out to a separate,
smaller Xapian database.
I've reproduced the problem on both Debian 8.x and 9.x with
Xapian 1.2.19 (chert backend) and 1.4.3 (glass backend)
respectively.