This code adds an iomgr implementation that's backed by an EventEngine. This uses the EventEngine API alone, and separate work will introduce an EventEngine prototype to plug into it.
See also drfloob#1: @nicolasnoble has a pull request against this branch, implementing the libuv-based EventEngine. One goal here is to implement the iomgr code such that it can be merged independently without affecting normal builds.
This implementation can be built using bazel build --cxxopt='-DGRPC_USE_EVENT_ENGINE' :all
Some shortcuts are being taken to get a working, testable version of the engine. EventEngines are not pluggable, for example.
Craig Tiller