Externalize tablet transactions

This commit introduces a framework for externalizing tablet transactions
to sibling servants. The feature introduced in this commit is not
self-contained and is only used in the later smooth tablet movement
commit.

yt/yt/client/object_client/public.h

@@ -103,6 +103,9 @@ DEFINE_ENUM(EObjectType,
     ((UploadNestedTransaction)                      (  8))
     ((SystemTransaction)                            (  9))
     ((SystemNestedTransaction)                      ( 10))
+    ((ExternalizedSystemTabletTransaction)          ( 11))
+    ((ExternalizedAtomicTabletTransaction)          ( 12))
+    ((ExternalizedNonAtomicTabletTransaction)       ( 13))
     ((TransactionMap)                               (407))
     ((TopmostTransactionMap)                        (418))
     ((LockMap)                                      (422))