Add asynchronous RPC support to co::Coroutine rpc server

[dallison]

Summary

• Reworks the co::Coroutine-based rpc server so non-streaming methods run as a two-coroutine pipeline: a request coroutine dispatches incoming requests to the handler, and a response coroutine drains completed replies from a toolbelt::SharedPtrPipe and publishes them to the client.
• Handlers now receive reply/error callbacks and may complete their reply later (e.g. after yielding/sleeping on their coroutine), decoupling request dispatch from response delivery.
• Adds the public RegisterMethodAsync(...) API. Existing synchronous RegisterMethod overloads are now thin wrappers over it, preserving prior behavior (including the "Error executing method <name>: ..." error formatting).
• Reply writes pass the request coroutine to SharedPtrPipe::Write, so a momentarily full reply pipe yields the coroutine (letting the response coroutine drain it) instead of blocking the scheduler thread or silently dropping the reply.

Design notes

• The whole server runs on a single coroutine scheduler thread, so no locking is used; replies are produced on the request coroutine that owns them.
• The response coroutine dups the interrupt fd so it can wait on both the reply pipe and shutdown without violating the epoll "one waiter per fd" restriction.

Test plan

• bazelisk test //rpc/... — all pass (client_test, server_test, rpc_test)
• New ServerTest.CallAsyncMethod exercises the async reply path (handler defers its reply via coroutine work before completing)