Use OOP for db api

We would like to use more OOP in more parts of rally.

For db API :
https://github.com/stackforge/rally/blob/master/rally/db/api.py.

We would like to have abstract class with abstract methods instead of a bunch of methods.

Steps to achieve this goal：

1. In the implementation:
    https://github.com/stackforge/rally/blob/master/rally/db/sqlalchemy/api.py.

    Create from these methods class:
    https://github.com/stackforge/rally/blob/master/rally/db/sqlalchemy/api.py#L63-L317
    "class Connection(object): " and all these methods

    And fix this line:
    https://github.com/stackforge/rally/blob/master/rally/db/sqlalchemy/api.py#L60
    return Connection()

    This should work out of box

2. Remove the import line in:
    https://github.com/stackforge/rally/blob/master/rally/db/__init__.py

    Things need to be done:
    a.
    b
    c.

    The reason to do this:
    a.
    b.
    c.

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We can make 1 class and put all these methods as an abstract .

And in the implementation:
https://github.com/stackforge/rally/blob/master/rally/db/sqlalchemy/api.py.
It should be actually subclass of this interface.

The whole change would be separated into two steps:

1. create class and put only these methods :
      https://github.com/stackforge/rally/blob/master/rally/db/api.py#L50-L62
      And, in implementation do the same.

2. put all methods into the class

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