The intent of this lib is not to replace fidl-async, but rather to be used by sysmem until both fidl-async and fidl-async-2 are essentially replaced by a lib usable from Zircon that can serve FIDL using FIDL C++ generated code (at which point sysmem and other FIDL-serving code in Zircon can switch directly to that). That'll be better than both fidl-async and fidl-async-2 (easier to correctly deal with handle lifetimes, easier async txn completion, etc).
In contrast to fidl-async, fidl-async-2 will continue to receive messages from a FIDL channel while a previous request received on that channel is asynchronously in progress. While fidl-async is able to process requests concurrently only if the requests are from separate channels, fidl-async-2 can concurrently process multiple requests received from the same channel as well. A per-binding concurrency_cap setting prevents this from resulting in unbounded build-up of in-progress requests (however, this is not back-pressure on the channel - hitting the concurrency cap intentionally fails the binding, to avoid the possibility of getting stuck due to the concurrency cap).
SimpleBinding<> is analogous to C++ FIDL's Binding<>.
FidlServer<> is a base class for FIDL server sub-classes. The FidlServer<> base class manages the SimpleBinding<> and thread-safe shutdown of the FIDL server.
FidlStruct<> can be used to automatically close all the handles in a C FIDL struct when a FidlStruct<> instance goes out of scope.
All of this functionality should be superseded by usage of FIDL C++ generated code in Zircon.