Change the place and resulting object for the methods of `readSql<-->Schema`

[zaleslaw]

Motivation:
All these objects are placed on the DataFrame, but return not a DataFrame, but schema, also we are limited with this schema from DataFrame, but it could not keep, for example, the name of the table and other meta-parameters.

Solution:
Move these bunches of methods to the helper class SqlSchema and introduce static methods on it, this class could be extendable and also reimplemented for any particular Database