Split the chained inject_parameter().value_or() expression into two
separate named variables for clarity.
Use condition_variable::when() instead of wait(). when() is the
coroutine-native API, designed specifically for co_await contexts —
it avoids the heap allocation of a promise_waiter that wait() uses,
and instead uses a stack-based awaiter.
Closesscylladb/scylladb#28824
fb40d1e411