HeaderManager instances could be reused across threads

[asfimport]

@vlsi (Bug 66269): It is unlikely that HeaderManager would be mutated during test plan execution, so adding NoThreadClone would reduce heap utilization when the test plan uses many HeaderManager elements (e.g. HeaderManager for each HTTP Sampler)

I've submitted the PR and the tests pass with the change: https://github.com/apache/jmeter/pull/727

Severity: normal OS: All