Fix bug when indicating if pr is mergeable.

Instead of using merge_commit_sha, which has buggy behavior, since it
indicates non mergeable pull requests as mergeable, we can use mergeable
value for more accurate representation. This solution requires one
additional request to get each pull request by itself.

Also, class `ProxyView` of flask app has been refactor to make one
access to memcache.
merge fix
more fixes for loadMergeableStatus()