Implement keepalive context manager and keepalive_mode. Fix #31

[penguinolog]

What do these changes do?

keepalive method for context manager and keepalive_mode flag
Subprocess runner switched on import concurrent.futures like SSH.

Are there changes in behavior for the user?

ssh context manager will close connections on exit, if it enforced by keepalive_mode == False.

Related issue number

#31

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes

[coveralls]

Pull Request Test Coverage Report for Build 102

• 34 of 34 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.1%) to 99.688%

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Totals
Change from base Build 99:	0.1%
Covered Lines:	957
Relevant Lines:	960

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💛 - Coveralls