Add a function to find out if LSP server has replied with completion
matches. This is needed in case of using external completion engines
that expect asynchronous callback. LSP client simply acts as a source
of completion items through omnifunc (g:LspOmniFunc). Before calling
the omnifunc the second time g:LspOmniCompletePending can be checked
to see if LSP server is ready to provide completion items.