Allow concurrent updates to properties hash

Can we add a method that allows the caller to add or overwrite a key/value pair in a feature's properties hash? Can we get away with managing conflicting writes via last-in-last-out, or is there a better approach we can take?

No files, tests, or entry points are named. First locate how a feature's properties hash is updated, then determine the intended behavior for concurrent additions and overwrites, including whether last-in-last-out ordering is acceptable. Done means the update API and conflict semantics are defined and covered by tests.

Written by the indexing model from the issue text.