ohno: add a runtime switch for backtrace collection

[Vaiz]

Instead of relying on RUST_BACKTRACE env variable that must be set before service is started, it would be more useful to have an ability to enable/disable traces collection for ohno errors when application is running.

this potentially supersedes #199

[Vaiz]

this feature implies the use of globals to control the behavior of backtrace collection. To avoid issues when there are multiple instances of the a global variable introduced by semver incompatible versions of the same crate, I suggest to add a separate crate for globals with the goal of newer introducing any breaking changes in that specific crate. This crate should not be used directly by anyone, all necessary functionality will be reexported through ohno crate.