Remove &mut self requirement in TimerSubsystem::delay()

[jordanhalase]

It is safe to call TimerSubsystem::delay() with an immutable reference to &self as long as the subsystem is initialized. It otherwise breaks the ability to create timers. Closes #1268

[Cobrand]

LGTM! Please add this change to the changelog, then we'll be good to go.

[jordanhalase]

Added to "Unreleased" section

[jordanhalase]

I'm not entirely sure if changing a function's type signature from &mut self to &self is a breaking change but Rust definitely complains about unnecessary mut tags on variables.

[Cobrand]

Thanks!
It's not really a breaking change unless some people have a deny(warn) in their projects, but it doesn't matter that much either way.