Improve session usage

[alecandido]

From SQLAlchemy:

Is the session thread-safe?

The Session is very much intended to be used in a non-concurrent fashion, which usually means in only one thread at a time.
The Session should be used in such a way that one instance exists for a single series of operations within a single transaction. One expedient way to get this effect is by associating a Session with the current thread (see Contextual/Thread-local Sessions for background). Another is to use a pattern where the Session is passed between functions and is otherwise not shared with other threads.

I.e.: instead of passing around an instantiated session, we should pass around a class Session, created by an initial sessionmaker call, and use it in this way:

from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker

# an Engine, which the Session will use for connection
# resources
engine = create_engine('postgresql://scott:tiger@localhost/')

# a sessionmaker(), also in the same scope as the engine
Session = sessionmaker(engine)

# we can now construct a Session() and include begin()/commit()/rollback()
# at once
with Session.begin() as session:
    session.add(some_object)
    session.add(some_other_object)
# commits the transaction, closes the session

This is not that complicated, and essentially it boils down to change this call:
https://github.com/N3PDF/banana/blob/c33d339fb5ce6bc483154a6a5589711d58e7a35b/src/banana/benchmark/runner.py#L209
removing the last call, to get just the session class, and then call Session.begin() every time that is need, directly inside the functions of the data/sql.py module.

(The overall idea, after changing the call, is to use the same "channels" that are currently used to propagate the session object, to propagate instead the Session class)