Received cancel notification for unknown message

Warning message emitted by the Language server of the following format:

`WARNING - pyls_jsonrpc.endpoint - Received cancel notification for unknown message id xxx`

Breaks completions and linting. Related Issues open on the core LS implementation:

https://github.com/palantir/python-jsonrpc-server/issues/17
https://github.com/palantir/python-jsonrpc-server/issues/50
