GH-3980: Fix concurrent initialization in DefaultSftpSessionFactory

[kdebski85]

fixes: #3980

Fix concurrent initialization in DefaultSftpSessionFactory

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[artembilan]

Thank you, but I have already made the fix here: #3981.
WDYT?

[kdebski85]

My approach might be slightly faster because:

1. I do not use synchronized
2. I do not use AtomicBoolean

Can I copy test from your PR?

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[artembilan]

@kdebski85 ,

any chances to see a unit test to confirm that we don't have concurrency issue any more?
As we discussed before you can simply copy-paste a test I provide in my rejected PR: https://github.com/spring-projects/spring-integration/pull/3981/files#diff-ea9ddb7293f89610654c162b3b862b3b590008c3feb48e12ec2d57c69027d16eR96

Thanks

[kdebski85]

@artembilan
I copied test from #3981.
Please review.

[artembilan]

@kdebski85 ,

thank you for contribution and reasonable discussion; looking forward for more!