Lets user have full control over how the Observable is created with
sane defaults for eager/non-eager cases.

- The fix adds a necessary buffer between the rx-rs adapter and
the timeout operator introduced in AbstractCommand. Unfortunately,
the hystrix operator does not comply with RS backpressure requirements.
It will always request an unbounded demand to a source regardless of
the command consumer demand. With this commit we protect RS adapter and
downstream flow from overflow. Ideally a rework of the operator should
be planned.

In order to fix issue with TPS in hystrix-dashboard threadpool section, removing average calculation for propertyValue_metricsRollingStatisticalWindowInMilliseconds. This was done for hystrixCommand.js previously but not for hystrixThreadpool.js.

- The eureka instance auto registration also changes the secure port when the server port is random (0)

Fixes gh-2303

Update documentation to reflect https://github.com/spring-cloud/spring-cloud-netflix/issues/1048