Remove thread unsafe statusCode and responseHeaders instance variables from apache http ApiClient

[aaronforest-wf]

fix #11852

This fixes the same problem as swagger-api/swagger-codegen#1435, but for the apache http client. It is implemented differently here to maintain backwards compatibility.

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[aaronforest-wf]

Ready for review

Java Apache Http template creators:
@harrywhite4 @andrevegas

Java technical committee:
@bbdouglas @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger @karismann @Zomzog @lwlee2608 @martin-mfg

[wing328]

to make this PR backward compatible, can we keep these changes and mark these methods as deprecated instead?

[aaronforest-wf]

Yes, I've re-added these methods. The implementations are a bit more complex now, but they should be thread safe.

[wing328]

thanks for the PR, which has been merged