Allow retransmission if it will result in faster delivery.

...va/org/briarproject/bramble/api/db/DatabaseComponent.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/Migration39_40.java

.../org/briarproject/bramble/sync/DuplexOutgoingSession.java

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

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

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