Detect deleted (or moved) messages in open folder: #3793

This commit finishes the second half of #3793 by detecting when messages have been deleted
(or moved out of) an open folder and notifying the system of the change.  The nonblocking
synchronization primitives have been beefed up and may be broken out to a separate library
some day.

This commit also introduces the ReplayQueue, which replays events that occur on the server
locally.  This class will probably become much more important as time goes on, perhaps used
to store user events that are replayed on the server as well.

src/engine/sqlite/abstract/sqlite-row.vala

@@ -7,7 +7,7 @@
 public abstract class Geary.Sqlite.Row {
     public const int64 INVALID_ID = -1;
     
-    private Table table;
+    protected Table table;
     
     public Row(Table table) {
         this.table = table;