(redo of 2999/I5609025feee8cfdecc09b55bfbf1bd13fa2e6602)

This PR is designed to bring more clarity within the docs by renaming object
instances that may be consfusingly similar to class, method, and attribute names.

For example, instances of the class `MetaData` are available on some  objects as
`.metadata` property, and had appeared within the docs as both `meta` and
`metadata` which has confused some users in the past. By this PR, the docs now
utilize the following naming convention:

* MetaData - SQLAlchemy class
* .metadata - SQLAlchemy API attributes
* metadata_obj - developer instantiated metadata objects or references

Detailed Changes:

* standardized `meta` and `metadata` instances to `metadata_obj`. note: the docs were evenly split between 'meta' and 'metadata'.
* standardized 'cursor' to 'cursor_obj' to avoid confusion with the method.
* standardized a 'scalar_subquery = ' to 'scalar_subq' to avoid confusion with the method.
* standardized a 'cte = ' to 'cte_obj' to avoid confusion with the method

Change-Id: I79c98aee16c5fc6649289b2dd7d6dfc368222fb4

this should be backported to 1.3 as well to as much a degree
as possible.

Includes a new recipe to set the default schema name
on connect.   this will only work on 1.4, but also requires
that we fix #5708 for it to work fully.

Change-Id: I882edd5bbe06ee5b4d0a9c148854a57b2bcd4741

Add SelectBase.exists() method as it seems strange this is
not available already.  The Exists construct itself does
not provide full SELECT-building capabilities so it makes
sense this should be used more like a scalar_subquery.

Make sure stream_results is getting set up when yield_per
is used, for 2.0 style statements as well.  this was
hardcoded inside of Query.yield_per() and is now moved
to take place within QueryContext.

Change-Id: Icafcd4fd9b708772343d56edf40995c9e8f835d6