OBMFULL-5259 removes last three warnings from source code
OBMFULL-5259 ported from OBMFULL-5253 make it possible to disable expunge globally on opush

Conflicts:
	java/sync/configuration/src/main/java/org/obm/configuration/EmailConfiguration.java
	java/sync/configuration/src/main/java/org/obm/configuration/EmailConfigurationImpl.java
	java/sync/configuration/src/test/java/org/obm/configuration/EmailConfigurationImplTest.java
	java/sync/opush/linagora-mail/src/main/java/org/obm/push/mail/imap/LinagoraMailboxService.java
	java/sync/opush/push-mail/src/main/java/org/obm/push/mail/MailBackendImpl.java
	java/sync/opush/push-mail/src/test/java/org/obm/push/mail/MailBackendImplTest.java
	java/sync/opush/push-mail/src/test/java/org/obm/push/mail/MailBackendTest.java
	java/sync/opush/push-mail/src/test/java/org/obm/push/mail/MailboxBackendTest.java