make default cache-busting key track more changes

[Galen Charlton]

The cache key that autogen.sh generates is based on checksums of generated fieldmapper, org unit tree, and locale files. While the contents of the fieldmapper file are very likely to change during major version upgrades, they are less likely to change during minor updates.

Now that the default expiration time for static assets is a year (per bug 1681095), this means that changes to static resources during minor version updates are less likely to be seen immediately by public catalog users. While bug 1681095 also added a mechanism by which the Evergreen admin can manually change the cache key, it would be better if the generated cache key took additional values into account, including (but not limited to):

- full version number
- hash of the tip of the Git branch from which the release was built
- hash generated from checksums of files in certain directories known to contain static content
- a purely random value (but in this case, it should somehow be arranged for the cache key to be the same across all app servers in a multi-server environment)

Evergreen master