Implement async

[trestletech]

Following httpuv's implementation of async, we should add an async implementation to plumber endpoints/filters.

[schloerke]

Closed in #248

[jeffkeller87]

FYI, there is a low-level and somewhat artificial performance issue in the parallel package on Linux. In effect, it puts a 35-45ms floor on data transfers between parallel socket connections.

For most parallel use-cases this penalty isn't noticeable. But for use-cases like plumber it is often quite noticeable if you are looking to achieve industry acceptable levels of API performance (which you probably are if you're going parallel). This has also been brought up regarding the future package: futureverse/future#437

I'd love to see the plumber community press R-devel to implement the fix, since we are the sort of use-case affected. I unfortunately do not have the skills to contribute such a low-level change to parallel.

[meztez]

Even if they implement the fix, it would not impact plumber since plumber relies on libuv from httpuv package.

TCP_NODELAY would have to be enabled from httpuv library.

[jeffkeller87]

My bad. I saw references to promises and assumed the future package was involved.