etcd jobstore support for apscheduler 3.x

[Yan-Daojiang]

Recently, many people use etcd as their metadata hub, so I tried adding etcd persistent job storage for Apscheduler 3.x. I added the corresponding unit tests with 100% coverage, and they passed the tests locally.

[agronholm]

I wouldn't have entertained the idea of adding a new back-end to the 3.x series but this PR was exceptionally well prepared, so I'm giving it a pass. I left some minor comments which should be addressed before merging though.

[Yan-Daojiang]

Hi agronholm! Thank you very much for your review of my code and your valuable suggestions, I made some changes to them.
I noticed that your focus may be on 4.x at the moment, and some important changes have been made. I'd love to be able to make some contributions on 4.x in the future. Since pre-release 4.x is not currently in production, I only contributed to 3.x.

[Yan-Daojiang]

There seems to be a problem in executor. This should not be caused by the new back-end. Could you help to take a look?
When runing the test case test_broken_pool , the default memory storage should be used.

[agronholm]

There seems to be a problem in executor. This should not be caused by the new back-end. Could you help to take a look? When runing the test case test_broken_pool , the default memory storage should be used.

test_broken_pool is known to be flaky, and there's an open issue about it.

[Yan-Daojiang]

OK, thank you! I have no other questions.