Previously sharing a Timer/Notifier between threads (or just handing
its ownership to another thread) lead to a crash as they are
thread-specific.
This commit makes it so we can handle mutation (i.e. just deletion
or unregistering) in a thread-safe and lockfree manner.
Not a single client of this API actually used the event mask feature to
listen for readability AND writability.
Let's simplify the API and have only one hook: on_activation.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
