The processing log is sent when a command issued by a request is being
processed, if the request has not yet been responded. In particular,
the processing log of sbtReportResult is filtered out if the client has
already received a response.
The processing log severity is the lowest so that it can be ignored by
the BSP client.
Initial draft for bsp support.
This shows two communication pattern around BSP.
First, if the request can be handled with the build knowledge is readily available in `NetworkChannel` we can reply immediately. `BuildServerImpl#onBspBuildTargets` is an example for that.
Second, if the request requires `State`, then we can forward the parameter into a custom command, and reply back from a command. `BuildServerProtocol.bspBuildTargetSources` is an example of that since it needs to invoke tasks to generate sources.