Use docid instead of id in search table

We had previously included an 'id INTEGER PRIMARY KEY' column in the
MessageSearchTable, assuming it would get the same rowid alias treatment
as it does in non-FTS tables.  That assumption was wrong: it was being
created as a FTS column.  This fixes it so we use docid everywhere.

To fix the old incorrect docid values, we simply blow away the search
table and let the natural search table population process, which now has
the correct docid insertion code, fix the problem.

This also removes the id column from the search table creation SQL, but
this will only affect new users.  Upgraders will see an empty, vestigal
id column in their search table.  Since SQLite doesn't easily let you
remove columns, it's just easier to ignore the column than go through
all the work to fix it.
Merge branch 'search-docid' into wip/725929-db-opt