MONGOID-4680 implement cache_version for better Rails caching support

[jamis]

This PR adds a #cache_version method to Mongoid::Document instances, as well as to the has_many and embeds_many proxies, which brings Mongoid closer to API parity with ActiveRecord.

This also modifies #cache_key to better parallel how it works in ActiveRecord: if cache_version returns non-nil, cache_key will not embed the timestamp in the key. Instead, the timestamp is used as the version for the entry (which reduces key churn).

Note: this PR does slightly change the behavior of Mongoid when used with Rails caching (#cache_key will not embed the timestamp when #cache_version is non-nil, as mentioned). We discussed this as a team and do not believe this constitutes a breaking change; in the worst case, systems that depend on caching may find at least some of their caches invalidated the first time they deploy this change.

For systems where the new cache key format is a concern, you can "revert" to the original cache key format by defining a #cache_version of nil:

def cache_version = nil

# or, for older, less glamorous, more boring and verbose rubies:
def cache_version; nil; end