Allow messages to be marked as temporary.

...va/org/briarproject/bramble/api/db/DatabaseComponent.java

...ava/org/briarproject/bramble/client/ClientHelperImpl.java

...e/src/main/java/org/briarproject/bramble/db/Database.java

...va/org/briarproject/bramble/db/DatabaseComponentImpl.java

...c/main/java/org/briarproject/bramble/db/JdbcDatabase.java

...main/java/org/briarproject/bramble/db/Migration45_46.java

...oject/bramble/versioning/ClientVersioningManagerImpl.java

...org/briarproject/bramble/client/ClientHelperImplTest.java

...rg/briarproject/bramble/db/DatabaseComponentImplTest.java

.../org/briarproject/bramble/db/DatabasePerformanceTest.java

...st/java/org/briarproject/bramble/db/JdbcDatabaseTest.java

...t/bramble/versioning/ClientVersioningManagerImplTest.java

...arproject/briar/introduction/IntroductionManagerImpl.java

...r/privategroup/invitation/GroupInvitationManagerImpl.java

...va/org/briarproject/briar/sharing/SharingManagerImpl.java

...ivategroup/invitation/GroupInvitationManagerImplTest.java

...riarproject/briar/sharing/BlogSharingManagerImplTest.java