Deprecation Notice: Mixer in-process adapter model

As discussed in the Policies and Telemetry WG, we are deprecating the in-process adapter model for Mixer, beginning with the 1.1 release.

No new in-process adapters will be accepted into the codebase. Authors that have contributed in-process adapters are expected to transition these adapters to the out-of-process model for the 1.2 release. A conversion tool will be available for use (PR currently in review).

cc: @Pengyuan_Bian @kuat @geeknoid @mandarjog