Prevent pool to receive more than available processes on scaling

[ferdinandfrank]

As described in #897 a queue might currently scale down to 0 processes even though a minimum amount of processes per queue is configured.
I observed the behavior especially when one queue needs to process a lot of jobs over a longer period of time (system_function in the screenshot). In that case it sometimes receives all processes from other busy queues (low in the screenshot) which leads to the other busy queue (low) receiving 0 processes as all others are already busy even though the minProcesses in the config is set to 3.

To prevent this issue this pull request ensures that a queue will not receive more than the available processes when scaling up by considering the configured minProcesses and the amount of configured queues using the following formula:
[maxProcesses] - (([amount of queues] - 1) * [minProcesses])

This ensures that every queue has the configured amount of minProcesses available.

[taylorotwell]

Thanks for contributing to Laravel! ❤️