Fix up some async method aliases in KernelManager

[kevin-bates]

In preparation for the 7.0 release, I was perusing the changes to KernelManager and noticed some "internal" methods (i.e., methods that won't typically be overridden and are called from within "public" methods) had been changed to async but didn't have a backward-compatible "non-async" alias. As a result, applications that subclass KernelManager and override any of the following methods will not have their overrides called.

• pre_start_kernel()
• post_start_kernel()
• request_shutdown()

This pull request addresses this issue by introducing the appropriate aliasing thereby restoring the hierarchical call sequence.

Note that these methods were made async for AsyncKernelManager, but that will be called out in the 7.0 release notes - along with the other changes.

[blink1073]

LGTM, thanks! I kicked the downstream build assuming the failure was spurious

[kevin-bates]

Thanks @blink1073. Yeah, all the tests had passed within my fork prior to creating the PR, and was about to do the same.

[kevin-bates]

@davidbrochart - With these changes, I think the qtconsole update in jupyter/qtconsole#494 should be rolled back as these are more correct and preserve "synchronicity" in KernelManager's API that otherwise had been hybrid only with respect to these 3 updated methods. I'm sorry I didn't catch this prior to these updates.

[davidbrochart]

Right, I just run the qtconsole tests with jupyter_client master and with this PR they fail. Let's roll back jupyter/qtconsole#494.