crimson/common: specialize seastar::is_future<> for interruptible_future<>

crimson/common: specialize seastar::is_future<> for interruptible_future<>

this ensures that seastar helper functions are able to accept
interruptible_future<> in place of vanilla seastar::future<>.

for instance, if concepts is enabled, seastar::when_all_succeed()
has a set of constraits which include:

is_future<std::remove_reference_t<decltype(*i)>>::value

where i is the iterator passed to this function. so, to ensure
seastar::when_all_succeed() works with interruptible_future,
we need to teach the is_future<> to accept interruptible_future
as well.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>