Mtime == 0 is valid (see commits 7842142c3288 and 628daced19b8), so
remove any contradicting asserts.
Also allow a zero docId for anything but MTimeDB::put, and replace the
assert in put with a warning message. (E.g. deleting a 0 docId may be useful
for cleanup, as inserting it was possible in Release builds).
For actual errors print warning messages (instead of debug), but silence
any NOTFOUND conditions (empty ranges are a valid condition).