Overhaul quiescence, method callback timing, and login methods on the client.

- Data streamed from the server is quiesced on a per-object basis, not a global
  basis. We track which documents a method stubs modifies, and create individual
  snapshots ("server documents") of those documents rather than whole-Collection
  snapshots. Data writes from the server to documents not modified by stubs are
  applied immediately to the local cache; other writes are applied to the
  "server document" snapshots. Server documents are flushed to the local cache
  when all method stubs that wrote to the document have sent their "data write"
  message.  (We still do "full database" quiescence after a reconnect.)

- Instead of calling method callbacks as soon as the result is received, we wait
  until all data that precedes their "data done" message is flushed to the local
  cache. This way, method callbacks can see all of their results locally. (This
  applies to Collection mutator callbacks as well.) If this delay is
  unacceptable, you can also specify the onResultReceived option to
  Meteor.apply; this callback is given the method result as soon as it comes in,
  and there's no guarantee that the local cache is up to date.
  (This is a client-only change: server-side callbacks do not block on the
  write fence.)

- Methods invoked with the "wait" option to Meteor.apply now wait until all
  preceding methods are fully finished to be *sent*, not just to call their
  callbacks. ie, previous calls block the "wait" method in the same way that
  "wait" methods block subsequent calls.

- Remove Meteor.userLoaded and {{currentUserLoaded}}.
  Meteor.userId() is now set only at the point where Meteor.user() is fully
      loaded.
  Current user data is published via an unnamed subscription, not via
      "meteor.currentUser".
  Replace them with Meteor.loggingIn() and {{loggingIn}}, which become true
      as soon as the login method is sent (instead of only once it succeeds).
  In accounts-ui, move the spinny into the dropdown, because it now shows up
      before error messages would.

- Previously, if we received the "result" message from a method but no "data"
  message, and then disconnected and reconnected, quiescence would be
  permanently blocked. Now, not only do we allow the app to continue working,
  but we even guarantee that the method's callback will be called at the
  "reconnect quiescence" point.

- Remove reset function from the Store API (the interface between
      _LivedataConnection and Collection), and add a boolean "reset" argument to
      beginUpdate instead.
  Add saveOriginals/retrieveOriginals functions to the Store API (pass-through
      to minimongo implementation).
  Allow "replace" messages to be passed to the Store API's update function
      (in addition to set/unset).
  Allow Store API implementations (eg tinytest_client) to not specify all
      functions.

- Server-side tinytest results now stream into the result page instead of
  appearing all at once at the end.

- Rename fields and methods of Meteor._LivedataConnection as camelCase, and
  prepend all internal fields with _.

- Different Meteor._LivedataConnection objects now have separate
  _userIdListeners _ContextSets.

- Remove snapshot/restore functionality from Minimongo collections. (Individual
  queries still have result snapshots.) The "server documents" in
  Meteor._LivedataConnection serve the equivalent purpose.

- Meteor.loginWithToken's callback is now a "call with error on error, call
  with no args on success" callback like the other login callbacks.

- The test-only Meteor._LivedataConnection.onQuiesce function is removed.
  Every single use of it is now supported by normal method callbacks.