Instead of having callers use get_timeout(), then compare it against the
current time, set up a timeout timer in the permit, which assigned a new
`_ex` member (a `std::exception_ptr`) to the appropriate exception type
when it fires.
Callers can now just poll check_abort() which will throw when `_ex`
is not null. This is more natural and allows for more general reasons
for aborting reads in the future.
This prepares the ground for timeouts being managed inside the permit,
instead of by the semaphore. Including timing out while in a wait queue.
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