feat(trace): add abstract Clock trait supporting custom timing impl

[Saplyn]

Motivation

Adding a way to allow user to supply their own Clock (time-recording) implementation if the default one (from std) isn't desired. Enabling deterministic testing with fake clocks and WASM-compatibility if the user has a WASM-compatible clock implementation.

Closes #657

Solution

Added an abstract Clock trait for timing, replacing the original implementation.

I decided to go with static dispatch (despite @jlizen suggest otherwise) because adding a dyn Clock would make trace layer and service non-copy, resulting a potential breaking API change.

TODOs

• test cases covering the changes
• whether Clock::now() and Clock::elapsed() should take &self or &mut self