fix: make broadcaster sink interruptable

What does this MR do?

Related to #1349

This is one of the MRs/changes required to address the problem outlined here and to enable graceful shutdown of registry which is part of #1349 (namely the implement proper shutdown for for Notifications-related goroutines).

I tried to be as much backwards-compatible as possible and to preserve the current behaviour while making the changes, even if it meant slightly more complicated code than it coulde. The new config parameter is kind of a compromise between "wait indefinitely" behaviour we have now and broadcaster being interruptible. We do not have reliable delivery of notifications ATM, so timing out after receiving SIGINT and droppping notifications is OK. What is more - k8s issues SIGKILL if registry does not terminate in time after SIGKILL so these notifications would be lost anyway.

NOTE: I rely heavily on go channels and goroutines that communicate as this is the idiomatic way in Golang to implement synchronization in an interruptible fashion.

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