common/async: add spawn_throttle for bounded concurrency with optional_yield

common/async: add spawn_throttle for bounded concurrency with optional_yield

a primitive for structured concurrency with stackful coroutines from
boost::asio::spawn(). this relies on spawn()'s support for per-op
cancellation to guarantee that the lifetime of child coroutines won't
exceed the lifetime of their spawn_throttle, making it safe for children
to access memory from their parent's stack

by taking optional_yield in the constructor, spawn_throttle transparently
supports synchronous execution (where optional_yield is empty) and
asynchronous execution within a stackful coroutine (where optional_yield
contains the parent's yield_context)

Signed-off-by: Casey Bodley <cbodley@redhat.com>