* refactor ConnectionFactory to static inner holder Singleton

* remove redundant import

* make getSqlSessionFactory() method private

* fix sonar issue

* remove @Ignore of MailUtilsTest

* add MailUtilsTest to pom.xml to test

* fix MailUtilsTest path error in pom.xml

* modify test method name

* add assert and logger to unit test

* add log

* add log

* add AlertDaoTest

* move AlertDaoTest to new module

* modify test in pom.xml

* remove unnecessary log