Fix Model.count base implementation and Normalize/validate the query filter

[raymondfeng]

The PR fixes two issues:

1. Fix the count() base implementation using buildWhere() from the sub-class
2. Add logic to normalize/validate the query filter object before delegating to connectors

/to @ritch
/to @bajtos (We'll need to merge them into the loopback-connector repo).

[bajtos]

The best way for documenting private methods is to use

/**
 * (...)
 * @private
 */

That way IDEs like WebStorm can use the type information provided by jsdoc (thanks to /**), but the documentation is discarded by strong-docs (thanks to @private).

Comments starting with /*! are usually used for license boilerplate that should be preserved by minifiers in the output bundle.

[bajtos]

@raymondfeng

We'll need to merge them into the loopback-connector repo.

Makes sense. Could you please review/approve loopbackio/loopback-connector#2 and submit a pull request with the changes you made to lib/sql.js?

BTW #127 is going delete lib/sql.js, I am not sure how much sense does it make to fix it in this patch.

[ritch]

Is this required to remove from strong-docs? I'm pretty sure @private is supported.

[ritch]

Minor comments from me. Otherwise looks good 👍.