crimson/common: extract parallel_for_each_state out

crimson/common: extract parallel_for_each_state out

if `parallel_for_each_state` is defined as a nested class in errorator,
clang fails to compile it:

../src/crimson/common/errorator.h:716:47: error: no class named 'parallel_for_each_state' in 'errorator<AllowedErrors...>'
    friend class errorator<AllowedErrors...>::parallel_for_each_state;
                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^

and the forward declaration does not help. so we have to extract it
out of the errorator. to speed up the compilation, it is moved into
errorator-loop.h. its name mirrors `include/seastar/core/loop.h`.

we could extract the `errorator<>::parallel_for_each()` out as well,
as its return type can be deduced from the type of Iterator and Func.

Signed-off-by: Kefu Chai <kchai@redhat.com>