Speed up the method for expiring sessions

[dnesteryuk]

Elixir provides a very simple and powerful mechanism to execute code in parallel https://hexdocs.pm/gen_stage/Experimental.Flow.html#content. This library can be used to improve the Sirko.Session.expire_all_inactive method.

[dnesteryuk]

The current solution takes ~35 sec to expire 1000 sessions with 10 visited pages. It is fine for 0.0.1 release.

Definitely, the flow should help to scale it. But, there is an issue with deadlocks on the Neo4j side. They appear when a few queries access same relations at the same time (an experimental branch is here). In order to use the flow, the current logic of expiring sessions has to be changed.

[dnesteryuk]

Having solved the N+1 query problem, the solution was drastically speeded up. Now it takes ~1 sec to expire 1000 sessions with 10 visited paged.