Add session persistence

[dguerri]

Besides improving performances, this change will make it possible to
use scp as a "drop-in" replacement to paramiko sftp.

[remram44]

This means that we must now call close() explicitely if we don't use the context manager.

Should get() and put() close the session if it was not already open?
Or do we just not care about leaving sessions open until the GC gets to them?

(also: we usually write 'persistence')

[dguerri]

Hi @remram44.
I have fixed the grammar issue in the commit message. Thanks.

About get() and put(), you are right. If the user forget to close() SCP session, this will be eventually done by the GC.
Using a with statement, that is done by __exit__() method.

This PR should be backward compatible and it just introduces a small performance improvement.

[remram44]

The only thing I see here, is that if we consider that the GC close()ing is fine, we don't need the context manager either.

[dguerri]

Not sure I get what you mean by saying that GC close()ing is fine.
If we are worried about user leaving session opened, we can rest assured CG will take care of that. Nevertheless, releasing resources as soon as they are no longer needed, is always a good practice.

[jbardin]

Hi @dguerri,

This looks good.

Can you just drop the __version__ line before I merge this? (I'll bump the version to 0.10.0 with a tagged release)

[dguerri]

Sure, thanks!