[feature/multiple-keyspace] Multiple keyspace support.

[slowquery]

hello.
In my project, I had to support multiple keyspaces.
but, still #225 #98 issue is not solving.
I solved it with minimal changes.

To use a different keyspace, a new object must be created.
At this time we cannot support multiple keyspaces because we are using the same object.

I want my Pull Request to be helpful, and I hope it will be updated.

Changelog

bindAsync function interface

bindAsync(options): Promise<void>;

change new ExpressCassandra object instance return

bindAsync(options): Promise<ExpressCassandra>;

Remark

expressCassandra.js line 125, 126 exists for the test case