Start Pool by default + Refactoring

Enqueues with canceled context now returns `ctx.Err()` instead of Err…

…JobCanceled

Signed-off-by: Sherif Abdel-Naby <sherifabdlnaby@gmail.com>

Use my forked semaphore implementation

Since I found a bug in `github.com/marusama/semaphore` Issue #3 that had a potential deadlock which was avoided/worked-around. however after submitting a PR and fixing the said bug the performance was halved.

Now the `x/sync/semaphore` is much better performance-wise however it is not resizable, hence use my forked resizable semaphore.

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