sql.py: DbConn.with_savepoint(): Profile (nested) transactions so that the run time for groups of commands (e.g. csv2db INSERTs) is known

csv2db: verbosity defaults to 3 so that detailed queries with profiling stats are included in the log file, to assist in optimization

csv2db: Don't cache per-row INSERT queries because this bloats the cache (there aren't repeated identical INSERTs that shouldn't be re-run like in row-based import)

sql.py with_explain_comment(), DbConn: Fixed bug where with_explain_comment() was being run in per-row imports (row-based import and csv2db with INSERT), causing the overhead of an EXPLAIN query for every single INSERT and filling up the cache with EXPLAIN query results, by adding autoexplain mode, only running with_explain_comment() in autoexplain mode, and only enabling autoexplain mode for column-based import

db_xml.py: put_table(): Turn on autoanalyze mode to help the query planner avoid sequential scans on tables that now contain data. (Don't do this in row-based import because it creates too much overhead per insert.)

sql.py: Run all EXPLAIN queries with log_level=4 since the EXPLAIN information is now usually generated when the query is generated rather than when it's run, so the log_level is not known

sql.py: Added with_explain_comment() to query generating functions so that nested queries will also have EXPLAIN information

sql.py: Added with_explain_comment() and use it in run_query()

sql.py: run_query(): EXPLAIN output: Run explain() with log_level 1 higher than the query's log_level, so that low-level queries' EXPLAIN queries are not output when the queries themselves are not output. This also ensures that only level 2 (major) queries have the EXPLAIN logged (to introduce the query that is being run), to avoid cluttering the log output.

sql.py: explain(): Support custom log_level