[DBTablesMonitoring] Add a method: getDBAgent().

This patch is to replace DBCLIENT_TRANSACTION_BEGIN()/END() into
DBAgent::runTransaction() as smooth as possible.
[DBTablesMonitoring] Use a new code fashion to do transaction.
[DBTablesMonitoring] Use a new code fashion to do transaction.

This commit replaces methods that call addTriggerInfoWithoutTransaction().
[DBTablesMonitoring] Use a new code fashion to do transaction.

This commit replaces methods that call addEventInfoWithoutTransaction().
[DBTablesMonitoring] Use a new code fashion to do transaction.

This commit replaces methods that call addHostgroupInfoWithoutTransaction().
[DBTablesMonitoring] Use a new code fashion to do transaction.

This commit replaces methods that call addHostgroupElementWithoutTransaction().
[DBTablesMonitoring]  Add a comment.
[DBTablesMonitoring] Use a new code fashion to do transaction.

This commit replaces methods that call addHostInfoWithoutTransaction().
[DBTablesMonitoring] Remove unnecessary empty lines.
[DBTablesMonitoring] Use a new code fashion to do transaction.

This commit replaces methods that call addItemInfoWithoutTransaction().
[DBTablesMonitoring] Use a new code fashion to do transaction.

This commit replaces methods that call addMonitoringServerStatusWithoutTransaction().
[DBTablesMonitoring] Use a new code fashion to do transaction.

This commit replaces a method that calls addIncidentInfoWithoutTransaction().
[DBTablesMonitoring] Use a new code fashion to do transaction in pickupAbsentHostIds().
Merge pull request #471 from project-hatohol/transaction-db-tables-action

Transaction db tables action