sql: suppress ordinary errors

[jsha]

During times of high load, we log a large number of error log lines like these:

[mysql] read ... i/o timeout [mysql] unexpected EOF

These go through the mysql log adapter in cmd/shell.go. We can log thousands of these per minute, which suggests they would be better as a stat than as a log line. We should suppress these in the log adapter and add a stat for them (assuming there is no stat we can already gather).