xapian: fix some compile warnings
[IRIS-2511] Preserve text section boundaries

when passing text to be indexed or snippetted down to Xapian.

This required changing the semantics of begin_part() so that multiple
begin_part()/end_part() pairs with the same part number in the same
messages are allowed.

Also, search_xapian.c now stores temporary text in a new way which preserves
those boundaries instead of squashing them, then passes the boundaries
down to Xapian using increase_termpos() for both indexing and
snippetting.

Unlike what I said in the bug, this should help us generate correct
snippets even without a re-index.
[IRIS-2513] fix ctl_conversationsdb -b without breaking sync

Also fixed -z to actually clear records to allow easier testing of -b.
squatter: rolling mode is incremental updates