Switch to AtomicU64 counter

[hlb8122]

Justification

• AtomicU64 has better performance than Arc<Mutex<u64>>.
• The fetch_add fits nicely here.
• last_nonce doesn't really make sense here, because you get 0 just after initializing the client (before you have used any nonce). next_nonce however breaks the API.

Notes

• AtomicUsize is more portable. https://doc.rust-lang.org/stable/std/sync/atomic/index.html#portability

[apoelstra]

Curious about the motivation for closing

[hlb8122]

@apoelstra No idea, I forgot I had this PR when I deleted the branch - perhaps that did it. I can restore it if you'd like.