Use is_plan from bluesky utils

[jwlodek]

Description

Switch to use new is_plan function from bluesky utils.

Motivation and Context

Solves #305 . Requires bluesky/bluesky#1780 to be released, and pinned in the requirements file.

Summary of Changes for Release Notes

Remove is_plan function, use instead the equivalent from bluesky utils that can handle the decorated functions.

Fixed

Bluesky builtin plans that use the @plan decorator are now correctly identified by queueserver.

[dmgav]

In my opinion, the internally defined function is_plan should not be deleted, but instead modified to call the function defined in bluesky. We also need to support older versions of Bluesky that don't have is_plan function by either checking the version of bluesky or using internal implementation if import fails. Also, bluesky is optional dependency, so is_plan should be imported inside the is_plan function. I would also be useful to add some tests to make sure the plans are actually recognized in the future.

[tacaswell]

I agree with @dmgav , this should be done as

try:
    from bluesky.utils import is_plan
except ImportError:
    def is_plan(...): ...

or we need to pin the version of bluesky qs depends on to be the first version that has is_plan (which is too aggressive in my view).