This adds some synchronization to the maps at the root of the API to avoid duplicated calls to the actual GH REST API.  Specifically this is targeted around the two maps, orgs and users.  This fix makes the GHPRB jenkins plugin behave much better when there are lots of projects that could build for a specific repo (even if only a few are actually triggered)

There are also a few fixes around GHUser and GHPullRequest
* GHPullRequest was checking a field that may be null (merged_by) when determining whether to fetch details.  An unmerged PR would make a bunch of Github API calls for each property accessed.
* Where GHUser was returned in various objects, we weren't going through the caching mechanism at the root, so calls to APIs on GHUSer often resulted in new REST calls.  Instead, return from the cache wherever possible.