implement a lock per entity and enforce a sequential consistency model for CRUD operations

the openwhisk api does not return a `revid` upon get. this means that updates against a resource may silently fail -- e.g. both updates from thread T1 and thread T2 will succeed (as indicated by openwhisk), but one or the other updates will not be visible in the steady state.

solution 1: expose the couchdb revid
solution 2: expose it, but call it something else (e.g. semver)
solution 3: more fully reify the existing `version` field to serve this purpose

along these lines, we should consider whether to only allow updates without a revid when passed a `force` parameter, and also how to document this lack of thread safety