Support Lambda Managed Instances

[bnusunny]

AWS just launched Lambda Managed Instances What's New. It supports multiple concurrent requests on a single execution environment. We should support it to get the benefit to the Rust Runtime users.

To support multi-concurrency, we can spawn a new async task for each invoke. Both lambda-runtime and lambda-http crates need new run methods because the tower service instances need implement Clone trait. Adding Clone trait to the exiting run methods will be a breaking change.

X-ray trace header propagation should not use the environment variable. We need use a thread-safe way to store the x-tray trace header for each request.

[jlizen]

Re:

we can spawn a new async task for each invoke

(This is assuming this feature is only going to be implemented for tokio, initially).

Just as an implementation suggestion, we probably should make sure that we aren't dispatching work from the tokio block_on thread (aka non-worker thread), but rather from a worker thread (ie inside a task).

We don't control whether the caller awaits the lambda_runtime future on a worker thread or not, so it seems prudent to always spawn a task to drive the receive loop.

This is a performance optimization to avoid ping ponging the socket between the I/O driver on a worker thread, and the non-worker thread.

[jlizen]

Another thing to think about would be, how does this fit into turnkey graceful shutdown handling. The simple answer is, just fire the graceful shutdown once, globally, but I imagine the nicer thing might be some signaling to allow each accept loop to clean up its own state.

[jlizen]

new async task for each invoke

Correction (discussed offline): new task per event loop based on max concurrency, not per invoke