Shared pool

[ckoehn]

This pr introduces a shared connection pool, on the driver level, for all festung queries. Previously we were using one session per user/request (update: I think this sentence is wrong. We create one session per thread.) . This seems to be unnecessary overhead, because we are always connecting to the same festung host and can re-use the tcp connections to speed things up.

This pr also introduces a one-time retry behavior, because we are having quite some ConnectionError entries in sentry.

One additional optimization is to make the interface of the Connection backwards compatible by adding a session kwarg or accepting just **kwargs.

Any kind of input is highly appreciated 😉

[scruplelesswizard]

Not that I can validate the code, but the concept sounds solid! Thanks @ckoehn!

[dermorz]

Let's try this with a locally running festung used in the tests.

[berend]

Looks good!