Store the number of unresolved comments in change index

When changes are queried, we want to return the number of unresolved
comments with each change. This number should be pre-counted rather than
loading all the comments and counting each time.

We also add new search operators, which allow user to query by
"has:unresolved" and "unresolved:<RELATION><NUMBER>".

The 'unresolvedCommentCount' field of 'ChangeData' will be null if
'lazyLoad' is false.

Change-Id: I97c6aeb47db48f56f94c5d184dfa36c8d7868cc8
diff --git a/Documentation/rest-api-changes.txt b/Documentation/rest-api-changes.txt
index 6a78d9f..b0e64a8 100644
--- a/Documentation/rest-api-changes.txt
+++ b/Documentation/rest-api-changes.txt
@@ -5152,6 +5152,8 @@
 Number of inserted lines.
 |`deletions`          ||
 Number of deleted lines.
+|`unresolved_comment_count`  |optional|
+Number of unresolved comments. Not set if the current change index doesn't have the data.
 |`_number`            ||The legacy numeric ID of the change.
 |`owner`              ||
 The owner of the change as an link:rest-api-accounts.html#account-info[