Use gRPC everywhere (over the wire)

[jprochazk]

Right now all our SDK comms are handled using "smart channels", which support many different sources:

• "At rest" data
  • File
  • iframe web events
  • JS channels
  • stdin
• "In flight" data
  • HTTP
  • WebSockets
  • TCP
  • gRPC

JS channels and iframe events are web-viewer only, and stdin is CLI-only, so we can ignore those for the time being, as they'll always require side channels of some sort. Files read from the local filesystem or served over HTTP will soon be encoded using a custom stream-level protocol:

• Encode LogMsg using protobuf #8347

We want to switch everything under "in flight" over to gRPC, no more custom protocols there.

Short-term goal is to standardize everything by migrating all the different pieces to Protobuf & gRPC without really changing any of the semantics. Mid-term goal is to evolve the semantics to fix some long-standing issues.