Generate handshake keys on demand, store when DB is opened.

bramble-api/src/main/java/org/briarproject/bramble/api/identity/IdentityManager.java

@@ -38,7 +38,18 @@ public interface IdentityManager {
 
 	/**
 	 * Returns the cached local identity or loads it from the database.
+	 * <p/>
+	 * Read-only.
 	 */
 	LocalAuthor getLocalAuthor(Transaction txn) throws DbException;
 
+	/**
+	 * Returns the cached handshake keys or loads them from the database.
+	 * <p/>
+	 * Read-only.
+	 *
+	 * @return A two-element array containing the public key in the first
+	 * element and the private key in the second
+	 */
+	byte[][] getHandshakeKeys(Transaction txn) throws DbException;
 }