Made subscription id unique

[psg-19]

Fixes #136

Summary :-

• Added Uuid crate in cargo.toml
• made new_subscription_id() function to generate id using uuid crate and removed hardcoded value from mod.rs and downstream.rs
• added test to check id created is unique and in proper format.

@Priceless-P

[jbesraa]

hey @psg-19
Thanks for looking into this. While your solution is sound and provide a solution for the uniqueness of every id generated, we dont utilize that id between miner(machine) connections. When a machine reconnect we dont retrieve the previous session attached to that miner. Because of that, it feels a bit unnecessary introducing this change now before doing the above.

Ill keep this open for now, and if we plan to maintain mining sessions, we will merge it.