Convert CurrentUser to IdentifiedUser without casting

Add an asIdentifiedUser() method to CurrentUser that throws
UnsupportedOperationException if the user is not an IdentifiedUser.
This has a number of minor benefits:
 - Fewer extraneous parens required for writing
   ((IdentifiedUser) u).getAccount().
 - Slightly more descriptive error message "FooUser is not
   IdentifiedUser" rather than a ClassCastException when an assumed
   precondition does not hold.
 - Implementation may be slightly more efficient, by overriding the
   method to "return this" rather than an extra instruction for the
   cast. (Not benchmarked.)

By far the most common use of ((IdentifiedUser u) is to immediately
call getAccountId(), so add a similar method getAccountId() to
CurrentUser.

These methods still throw unchecked exceptions, which matches the
existing behavior. Some calls are guarded by isIdentifiedUser() and
take different behavior if false; this ranges from throwing a checked
exception, typically but not always AuthException, to substituting
GerritPersonIdent. Other calls just continue to assume that the user
is always an IdentifiedUser.

Change-Id: I2ab2028a7cdc0f703c4a4cdc2b5797d87ab7024c

gerrit-acceptance-framework/src/test/java/com/google/gerrit/acceptance/InProcessProtocol.java

@@ -326,8 +326,7 @@ class InProcessProtocol extends TestProtocol<Context> {
           throw new ServiceNotAuthorizedException();
         }
 
-        IdentifiedUser user = (IdentifiedUser) ctl.getUser();
-        rp.setRefLogIdent(user.newRefLogIdent());
+        rp.setRefLogIdent(ctl.getUser().asIdentifiedUser().newRefLogIdent());
         rp.setTimeout(config.getTimeout());
         rp.setMaxObjectSizeLimit(config.getMaxObjectSizeLimit());