crimson/common: bring green_get() to interruptor

crimson/common: bring green_get() to interruptor

The rationale
=============
Calling `get()` on an unavailable future can result in yielding
the reactor, so -- from the interruptor's POV -- the interrupt
condition must be handled exactly like it is done for `yield()`.

Interruptor takes care about that but, at the moment, only for the
interruptible futures; there is mechanism for plain `seastar::future`.
This patch paves the way for handling them as well.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>