feature: limitation on maximum pending notifications

See merge request !40

tests(scheduler): simplification of integration tests data initialization

See merge request !38

Service injection made the tests depend on the good implementation of
the waitUntilNoActivity method. This commit's tests (except the rate
limiting ones which measure the exact operation duration time) do not
depend on this method implementation anymore, and actually represent a
maximal tolerated time duration

Veuillez saisir le message de validation pour vos modifications. Les lignes

use RejectionReason in NotificationHandler instead of plain String
gauge representing total number of current pending notifications

🔟+1️⃣ tests(scheduler): refacto on last success/error field update verif

See merge request !34

2️⃣ review notification handler

See merge request !33

1️⃣ feature(scheduler): execution duration metric refacto

See merge request !31

1️⃣ chore(deps): upgrade pushy 0.14.1 → 0.15.1 & bucket4j 6.2.0 → 7.5.0

See merge request !30

APNSTemplate refacto:
* Removed semaphore to use AtomicInteger instead, way
lighter component
* Parallel APNS activity scouting in FailoverAPNSTemplate
* Various MR comments