Add a check of the session state before take a task

[LeonidVas]

We need to check of a session state before take a task after wait()
because session maybe in disconnecting state and task will be
hang in a take state after the session will be disconnected

Fixes #104

[olegrok]

LGTM. Some questions were discussed verbally

[Mons]

LGTM

[Totktonada]

I vote for canceling fibers, which will not be able to proceed with the task. We even asked recently to do this automatically: tarantool/tarantool#4788 (just for information).

I would also describe what is the root of the problem in the test case. Very sketchy it may look so:

Put task, take it, take again (no more tasks, it'll be blocked in tube.take()), disconnect, schedule the fiber with blocked take (but don't run yet) (from method._on_consumer_disconnect), release the task (from method._on_consumer_disconnect), <here the scheduled fiber with blocked take will be executed>.

The result is that the task will be in 'taken' state, however there is no consumer, which may work with it. The task likely will be stuck in this state until a server restart.

At least look whether you can improve the description and if so, spend a hour on it, please. (I often stuck with changes made several years ago w/o described reasoning, so maybe I overdramatize this, but anyway.)

[LeonidVas]

Updated

[LeonidVas]

Now two implementations have been presented, and you need to choose one.

[Totktonada]

Leonid said that he met problems with approach with fiber.cancel(). The reason likely is caching of fiber conds (see compat.lua) and that waiters field of struct fiber_cond remains filled after canceling the fiber.

Let's stick with the original Leonid's variant.

[Totktonada]

Pushed two small fixups.

[Totktonada]

Squashed them (Leonid agreed).

[Totktonada]

LGTM.