Prevent real time suggestions blocking editor

[intitni]

• Make the request interval shorter.
• The first request will trigger the XPC Service to provide a suggestion, but won't wait for it to finish. Instead, if will return immediately.
• The XPCService will cache the suggestion, if there is a future request asking for suggestions for the same code and cursor position, return the cache. Otherwise, treat it as a first request.

[intitni]

Done in 0.4.0