Issue 330: Support removeDocument from synchronizer and isDocSubscribedTo

[georgewsu]

Summary:
Add support for removeDocument from synchronizer so that we can start evicting doc handles from Repo handleCache.
Add support for isDocSubscribedTo so that we only evict doc handles that don't have any subscribed peers.

Issue:
#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:
#365

Part 2:
This PR only covers introducing removeDocument and isDocSubscribedTo so that in the next PR we can evict doc handles from Repo handleCache.