Add possibility to configure expected interval between clients' renew…

[mgtriffid]

…s and not break self-preservation

Currently, if one sets renewal interval different from 30 seconds on clients, self-preservation doesn't work correctly. For example, see spring-cloud/spring-cloud-netflix#373 . This PR adds possibility to configure expected renewal interval on Eureka server, so that if clients send renews every, say, 20 seconds, then SP threshold is 0.85 * 3 * count, not 0.85 * 2 * count.

[holy12345]

@mgtriffid Thanks your PR : )
Tiny things.
Could you please add some validate about serverConfig.getExpectedClientRenewalIntervalSeconds() is not zero.

[neoremind]

I think there might be a more aggressive solution to make heartbeats interval configurable. Because there are too many hard code by / 2 in class AbstractInstanceRegistry and PeerAwareInstanceRegistryImpl, if we set so then there is no need to add this parameter.

[qiangdavidliu]

@mgtriffid the PR mostly looks good, thanks. I think you may need to make a change here too:
https://github.com/Netflix/eureka/blob/master/eureka-core/src/main/java/com/netflix/eureka/registry/PeerAwareInstanceRegistryImpl.java#L294

Also, please address @holy12345 's comment and add some safeguards to the config. Thanks.

[qiangdavidliu]

This computation is repeated three times and might be better moved to a central method.

[mgtriffid]

I think you may need to make a change here too:
https://github.com/Netflix/eureka/blob/master/eureka-core/src/main/java/com/netflix/eureka/registry/PeerAwareInstanceRegistryImpl.java#L294

Sorry, I don't get it what do these two minutes mean. This seems to be an expiration check, but it uses lease duration and last lease renewal, and I can't understand why do we need to add 2 minutes. Could you kindly explain, @qiangdavidliu ?

Addressed other comments.

[qiangdavidliu]

@mgtriffid no worries, that was a minor issue. Thank you for the PR.