Fix a bug in calculating the average number of entries for keys not present in the sqlite_stat4 table.
Re-enable reading from the sqlite_stat3 table (as well as sqlite_stat4).
Handle a NULL input to decodeIntArray() that can result from a prior OOM.
If there is data in both the sqlite_stat4 and sqlite_stat3 tables for a single index, ignore the sqlite_stat3 records.
If ENABLE_STAT3 is defined but ENABLE_STAT4 is not, have ANALYZE create and populate the sqlite_stat3 table instead of sqlite_stat4.