Generate handshake keys when creating local author.

.../org/briarproject/bramble/api/identity/AuthorFactory.java

.../org/briarproject/bramble/identity/AuthorFactoryImpl.java

...rg/briarproject/bramble/identity/IdentityManagerImpl.java

...riarproject/bramble/identity/IdentityManagerImplTest.java

...ain/java/org/briarproject/briar/feed/FeedFactoryImpl.java

...java/org/briarproject/briar/test/TestDataCreatorImpl.java

...g/briarproject/briar/blog/BlogManagerIntegrationTest.java

...briar/introduction/IntroductionCryptoIntegrationTest.java

...arproject/briar/messaging/MessageSizeIntegrationTest.java

...test/java/org/briarproject/briar/test/BriarTestUtils.java