Add comment support to query builder

[ercanozkaya]

See: https://jacobbednarz.com/posts/tracking-sql-queries

Add a comment method to query builder to prepend comments to queries so that they show up in the logs. Example:

db_select('users', 'u')
  ->comment('module:my_custom_module,function:some_slow_function')
  ->fields('u', array('mail', 'status'))
  ->execute()

# => /* module:my_custom_module,function:some_slow_function */ SELECT u.mail AS mail, u.status AS status FROM users u

Now should the query pop up in your slow query logs, it will be annotated with some helpful debugging information on it’s caller.

# Time: 150517  6:26:05
# User@Host: mysql_user[mysql_user] @ db1.example.com [10.0.0.60]
# Thread_id: 4038366  Schema: example_table  Last_errno: 0  Killed: 0
# Query_time: 2.388451  Lock_time: 0.000041  Rows_sent: 0  Rows_examined: 1302  Rows_affected: 1302  Rows_read: 1302
# Bytes_sent: 13  Tmp_tables: 0  Tmp_disk_tables: 0  Tmp_table_sizes: 0
# InnoDB_trx_id: 2D57FA24A
use example_table;
/* module:my_custom_module,function:some_slow_function */
SELECT *
FROM users