[ENH] Cleanup Realtime Module

[teonbrooks]

The realtime module has been worked on piecewise and doesn't receive the same attention as other parts of the project. The aim would be to build upon #6120, #6141 with a more unified approach to our realtime clients and servers.

Todos

• [x] Make examples run in circleci (see https://github.com/mne-tools/mne-python/pull/6141#discussion_r275908523)
• [x] Get LSLClient and RtEpochs to play nice. The RtEpochs may need to be cleaned up in order to best serve other clients. I am having a difficult time debugging its interactions with the LSLClient.
• [x] Get the FieldTrip Realtime examples to run on CircleCI. this requires getting the external dependencies on circle.
• [x] Restore LSL realtime example on CircleCI
• [x] Make LSL stream loop over the file
• [ ] Make LSL client work with RtEpochs