Break BaseService functions into serviceutil

[brandur]

I was going through trying to refactor the demo project to use
rivershared, and while doing so, noticed that it'd broken out some
of the BaseService functions like CancellableSleep into a new
package so that they could be used without an available BaseService.

Looking over this API again, there's no good reason for functions like
CancellableSleep and ExponentialBackoff to be tied so tight to
BaseService. The reason they are is that BaseService provides a
random source that both can conveniently use, but that can be moved to a
function argument instead.

The reason that BaseService has a random source is to avoid the old
math/rand seeding problem. A single random source is initialized and
seeding securely once, then placed onto an archetype that's inherited
by all BaseService instances.

Soon, even that can go away. One of the biggest things fixed by Go
1.22's math/rand/v2 is that it's no longer necessary to seed the top
level functions. Once we drop support for Go 1.21, we'll be able to
simplify this code dramatically by dropping BaseService.Rand and the
random source arguments from functions like ExponentialBackoff in
favor of just using top level math/rand/v2 functions.

I also drop the variant CancellableSleepBetween in favor of having
callers use CancellableSleep combined with randutil.DurationBetween,
a new random helper similar to IntBetween.

With all this in, we'll be able to fully jettison all utilities from the
demo project in favor of going all in with rivershared's equivalents.

[brandur]

A workspace side effect: adding a new package to rivershared seems to make it hard to resolve by go get -t despite go test and another commands working fine. No functional change by removing this since all Go commands will get dependencies, but another strike against workspaces, and a weird one.

[brandur]

@bgentry With these changes I think we can get the demo app fully switched over to rivershared.

[brandur]

thx.