search: rename "ts:" prefix to "rt:"

search: rename "ts:" prefix to "rt:"

Meaning "Received time", as it is the best description of the
value we use from the "Received:" header, if present.  JMAP
calls it "receivedAt", but "rt:" seems like a better
abbreviation being in line with "dt:" for the "Date" header.

"Timestamp" ("ts") was potentially ambiguous given the presence
of the "Date" header.