Allow specifying waitfirst when waiting on a Condition (#47277)
* Allow specifying waitfirst when waiting on a Condition
I have a use-case where I use a `Condition` as a wait queue,
where a singler waiter is woken up on `notify`. Once a waiter
is woken up, it may need to re-wait, but we want it to get back
in the wait queue _first_ instead of the FIFO behavior of regular
`wait`. This allows the waiter to continue being notified until
a logical condition is met and the next waiter in the queue will
take the next notification.
This PR proposes adding a keyword argument `waitfirst::Bool=false` to
the `wait(c::Condition)` method that allows the caller to put itself
_first_ in the `Condition` wait queue instead of last.
I didn't consider whether other `wait` methods (Channel, Task, etc.)
would also benefit from something like this to minimize the total changes
to a specific use-case that I know is useful.
* change waitfirst -> first as suggested by jameson
