Add support for removing doc handle from handleCache

[georgewsu]

Summary:
Add support for removing doc handle from handleCache to free memory, without deleting document from storage.

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.

Next steps:
Once removeFromCache is added, it can be called from the Repo and/or synch server to free memory usage (that code must determine that the doc handle is safe to be removed from cache at that time).

[alexjg]

This looks good but could you add a typedoc @experimental tag to the removeFromCache method as I think it's quite likely we'll change this API a bit before we're sure of how the cache eviction stuff should work.

[georgewsu]

This looks good but could you add a typedoc @experimental tag to the removeFromCache method as I think it's quite likely we'll change this API a bit before we're sure of how the cache eviction stuff should work.

Sounds good, also added a check for documentId being present before proceeding to remove it.