Fix a number of database hiccups

1) 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.

2) Do as many rowid lookups as possible in batches, instead of doing
them individually in loops.  This speeds up working with large sets of
email.

3) Rejigger indices on the MessageLocationTable to make certain queries
faster.  This creates a new covering index in particular for the email
prefetcher, which previously had to sort using a temp table.  The new
index should work in the general case too, as we should never be looking
at ordering without folder_id (and since folder_id comes first, it works
as an index on just folder_id, too).

4) For bonus measure, log all slow queries (> 1s execution time) to
debug output.

Closes: bgo #725929

sql/version-020.sql

@@ -0,0 +1,7 @@
+--
+-- We had previously incorrectly included an id column in the search table.
+-- The code is fixed to use docid instead, so we just empty the table and let
+-- the natural search table population process make things right.
+--
+
+DELETE FROM MessageSearchTable