search: Add stable queries to thread search results

These queries will match exactly the set of messages currently in the
thread, even if more messages later arrive.  Two queries are provided:
one for matched messages and one for unmatched messages.

This can be used to fix race conditions with tagging threads from
search results.  While tagging based on a thread: query can affect
messages that arrived after the search, tagging based on stable
queries affects only the messages the user was shown in the search UI.

Since we want clients to be able to depend on the presence of these
queries, this ushers in schema version 2.