More intuitive interfaces for creating semaphores and channels. Also
allows channel construction using third-party bootstrappers directly
without overriding MSCCL++ Bootstrap.
* In cases when the same `tag` is used for receiving data from the same
remote rank, #514 changed the behavior of `Communicator::connect` and
`Communicator::recvMemory` to receive data in the order of
`std::shared_future::get()` is called, instead of the original behvaior
that receive data in the order of the method calls. Since the original
behavior is more intuitive, we get that back. Now when `get()` is called
on a future, the async function will first call `wait()` on the latest
previously returned future. In a recursive manner, this will call
`wait()` on all previous futures that are not yet ready.
* Removed all deprecated API calls and replaced into the new ones.
* Renamed `ProxyChannel` -> `BaseProxyChannel` and `SimpleProxyChannel`
-> `ProxyChannel`. It makes the interface more consistent by defining
channels to be associated with a certain src/dst memory region:
`ProxyChannel` as "sema + src/dst + fifo" and `SmChannel` as "sema +
src/dst". BaseProxyChannel is not associated with any memory regions, as
"sema + fifo".
* `ProxyChannelDeviceHandle` now inherits from
`BaseProxyChannelDeviceHandle`, instead of having one as a member.
