Keep connection alive (dealing with mysql wait_timeout)

These connection are used by dtle

incr

• applier
  • worker connection
    • sql.Conn: manually ping-ed
  • applier connection
    • sql.DB: db.SetConnMaxLifetime()
• extractor
  • slave thread: not sleeping
  • connection for meta info e.db/b.db

full

• applier
  • sql.DB
• extractor
  • sql.Tx: manually ping-ed

Currently ConnMaxLifetime is hard-coded to 300s for db.SetConnMaxLifetime

fixed in fb21089.

Now dtle should work properly if mysql wait_timeout is larger than 300. e.g.

set global wait_timeout = 320;