Fixing clippy warnings

I run clippy and the only complains are of the form: ``` warning: the following explicit lifetimes could be elided: 'a --> src/exec/workload.rs:47:6 | 47 | impl<'a> ToValue for SessionRef<'a> { | ^^ ^^ | = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#needless_lifetimes = note: `#[warn(clippy::needless_lifetimes)]` on by default help: elide the lifetimes | 47 - impl<'a> ToValue for SessionRef<'a> { 47 + impl ToValue for SessionRef<'_> { ``` Looking to existing code there are already cases where anonymous lifetimes are used. So fixing clappy's warning can be helpful.