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
81c4809ac3