Mongot will define a patch trigger alias to trigger creating a downstream/child patch on 10gen/mongod and we want to reuse the server and mongot binaries.

When the tasks in the upstream/parent patch on 10gen/mongot complete, mongot will save the relevant binary s3 tarball URLs as key/value pairs in via downstream_expansions.set() 

At the beginning of the compile script, the server will want to check for the existence of these keys in our expansions and if they exist, download and unzip the tarball and use those binaries instead of building new ones.