Manage Data and Buffer PVs in Buffer Object

[eloise-nebula]

In our current pattern:

1. we pass to a buffer to a device object
2. then that device calls buffer.get which calls epics.caget
3. We clean up HST buffers in epics.pv._PVCache_ that slac-timing was responsible for in the first place.

We could just manage those PVs ourselves. Really we could skip creating the devices altogether, but that's another project.