Enable shell feature for debug builds and disable shell feature for release builds

[jxyang]

As part of #292, we made shell feature always available to avoid pipeline failures. We should find another to disable it for release builds, using either build time or run time flags.

[paulcallen]

Only enable debug features in debug builds, not in release builds.

[bodzhang]

This experimental feature is disabled in release build. Changed the severity to low.