time,sync: make Sleep and BatchSemaphore instrumentation explicit roots

[hds]

Motivation

When instrumenting resources in Tokio, a span is created for each
resource. Previously, all resources inherited the currently active span
as their parent (tracing default). However, this would keep that parent
span alive until the resource (and its span) were dropped. This is often
not correct, as a resource may be created in a task and then sent
elsewhere, while the originating task ends.

This artificial extension of the parent span's lifetime would make it
look like that task was still alive (but idle) in any system reading the
tracing instrumentation in Tokio, for example Tokio Console as reported
in tokio-rs/console#345.

Solution

In #6107, most of the existing resource spans were updated to
make them explicit roots, so they have no contextual parent. However,
2. were missed:

• Sleep
• BatchSemaphore

This change alters the resource spans for those 2 resources to also make
them explicit roots.