Deprecate `SLWaitUntilTrue` and `SLWaitUntilTrueRetryDelay`. (resolves #241)

They have been replaced by `SLIsTrueWithTimeout` (and
`SLIsTrueRetryDelay`), to resolve confusion between `SLWaitUntilTrue` and
`SLAssertTrueWithTimeout` (#236).

Use of `SLWaitUntilTrue` and `SLWaitUntilTrueRetryDelay` now causes
warnings to be logged.