Fix session context corrupted problem

[PzaThief]

Fixes #[ISSUE NUMBER].

No related issue

Changes proposed in this pull request:
First of all, thank you to show nice sample. It was very helpful to build echo project.
But, I found a problem that session in container has shared struct "context".
It makes race condition and cause data error or concurrent map writes.
The first commit is POC of race condition error that never want it to happen.
If remove time.Sleep(1 * time.Second) in second goroutine, you may see also concurrent map writes sometimes.
So, I fixed it to do not use context sharing and use it as parameter.
Now there is a restriction of using session to use echo.context makes can not use in service layer, but there is no reason to access session in business logic, so it it not problem of this structure.
Please comment if you have any question about it.

[ybkuroki]

@PzaThief
Thank you for your pull request!
I think there is no problem your souce code so, I will merge into master.