Add support for clearing doc from DocHandle

[georgewsu]

Summary:
Add support for clearing doc from DocHandle so that reference can be released and memory freed, without deleting document from storage

Issue:
automerge#330

Background:
After creating enough documents in a repo, out of memory errors will cause a synch server to crash because memory isn't being freed. Repo holds references to each DocHandle in handleCache and each DocHandle holds a reference to the automerge doc.

Proposed solution:
https://github.com/georgewsu/automerge-repo/tree/gsu/cache-eviction-test2 has code for an initial implementation of cache eviction and releasing document reference so that memory can be freed. I've tested this with a sync server running locally.

Part 1:
This PR only covers introducing a new state and event to DocHandle to allow clearing the document.