raise if isolation_level execution option is used when a Transaction is present; warning in 0.9

[Mike Bayer]

isolation level settings typically do not work until the next transaction, and at least in the case of Psycopg2 are destructive. For this reason, the MySQL dialect already implements a COMMIT, and psycopg2 actually resets the transaction when you change the level:

>>> import psycopg2
>>> conn = psycopg2.connect(user='scott', dbname='test', password='tiger', host='localhost')
>>> cursor = conn.cursor()
>>> cursor.execute("create table foo (id integer)")
>>> cursor.execute("insert into foo (id) values (1)")
>>> cursor.close()
>>> from psycopg2 import extensions
>>> conn.set_isolation_level(extensions.ISOLATION_LEVEL_SERIALIZABLE)
>>> cursor = conn.cursor()
>>> cursor.execute("select * from foo")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
psycopg2.ProgrammingError: relation "foo" does not exist
LINE 1: select * from foo
                      ^

huge warning is needed and definitely an exception in 1.0.

[Mike Bayer]

this must include some approach to accommodate the user that is doing this:

session.connection().execution_options(isolation_level=...)

for that, we either need to get the warning to not emit if no SQL has yet been emitted, or provide some new pattern for the above.

[Mike Bayer]

• A warning is emitted if the isolation_level parameter is used with :meth:.Connection.execution_options when a :class:.Transaction is in play; DBAPIs and/or SQLAlchemy dialects such as psycopg2, MySQLdb may implicitly rollback or commit the transaction, or not change the setting til next transaction, so this is never safe.
• Added new parameter :paramref:.Session.connection.execution_options which may be used to set up execution options on a :class:.Connection when it is first checked out, before the transaction has begun. This is used to set up options such as isolation level on the connection before the transaction starts.
• added new documentation section detailing best practices for setting transaction isolation with sessions. fixes #3296

→ <<cset 9cbe235810b7>>

[Mike Bayer]

• A warning is emitted if the isolation_level parameter is used with :meth:.Connection.execution_options when a :class:.Transaction is in play; DBAPIs and/or SQLAlchemy dialects such as psycopg2, MySQLdb may implicitly rollback or commit the transaction, or not change the setting til next transaction, so this is never safe.
• Added new parameter :paramref:.Session.connection.execution_options which may be used to set up execution options on a :class:.Connection when it is first checked out, before the transaction has begun. This is used to set up options such as isolation level on the connection before the transaction starts.
• added new documentation section detailing best practices for setting transaction isolation with sessions. fixes #3296

(cherry picked from commit 9cbe235810b7c0c24d2556b4bb581b0207812e2d)

Conflicts: test/orm/test_bind.py

→ <<cset 280497cc5bda>>