SessionPool - locking down session to prevent concurrent usage.

[petrpatek]

Allow session to by lock and not picked for use. If the locked session is picked, SessionPool should pick a new one. It is useful when logging in, solving captchas.

[pocesar]

@petrpatek check if this use-case could be solved with #635 or how the flow could be interrupted / changed from the event itself

[B4nan]

Closing as stale, FYI we are planning some refactorings around session handling later this year which should help with this use case too (generating the sessions upfront and having more control over what is used for what, aka user pool).