Lock for API calls

[lorjala]

Add lock context manager and use it when allocating or releasing a device so no multiple api requests are sent at the same time.

NOTE: this is workaround for DeviceFarmer/stf#620 and it works only robust when running client on same machine. Parallel runs with different hots still potentially suffer with this bug.

TODO:

• unit tests

[coveralls]

Pull Request Test Coverage Report for Build 3599779302

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 15 of 17 (88.24%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.08%) to 83.22%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
stf_appium_client/tools.py	10	12	83.33%

Totals
Change from base Build 3581727136:	0.08%
Covered Lines:	367
Relevant Lines:	441

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💛 - Coveralls