implement a "baton IOC"

[tacaswell]

Write a (caproto) IOC to hold a "baton" string.

The idea is that:

• at init the bsui "takes the baton" by writing a unique key to the IOC
• at call time the RE (via a pre-processor) checks that it "still has the baton" (that is its key is still in the baton PV), if not fail to start
• provide a method that will "steal the baton" to get control of the beamline again

For complex beamlines with more that one end station shared hardware it may be worth generalizing to a baton per shared Device / endstation.

xref https://github.com/bluesky/bluesky/issues/17

attn @prjemian