add useWritePdo for select query
add onWrite method to let Model use write pdo for selecting
Fixed port separator in DSN when using dblib with PDO in sql server connector
adjust coding style
add docblock
Merge branch '4.2'
Clean up a few things.
Merge pull request #6638 from jlyu/select-from-write-pdo

[4.2] Select From Write Pdo
Fix method name.
Cleaning up some code.