feat: use parking_lot::Mutex

[jhgg]

parking_lot::Mutex is faster than std::sync::Mutex, so let's use it for the critical lock in CassFuture.

This section of parking_lot's docs describes the difference: https://docs.rs/parking_lot/0.11.0/parking_lot/type.Mutex.html#differences-from-the-standard-library-mutex

[kw217]

Is there a need for this? I just read faern/parking_lot#1 and rust-lang/rust#56410 and it seems like parking_lot is still a fair way off being considered for the standard library, due to various worries about correctness and developer documentation.

[jhgg]

The main motivation is that it's faster under the uncontended case (which is the majority here), and namely, our use of std::Mutex is incorrect anyways as we potentially can panic (although this is super unlikely) from a CFFI function which to my understanding is UB. (panic being unwrapping a poisoned mutex), https://doc.rust-lang.org/nomicon/ffi.html#ffi-and-panics

[jhgg]

anyways - we also run parking_lot in prod without any issue. so aside from a slight increase in compile time this should purely be "faster".

[kw217]

OK. I see lots of other projects are using parking-lot, and your point about unwrap in CFFI is a very good one.

Code LGTM - thanks for driving this.

[kw217]

As with the other ones, this is now out-of-date so needs you to click "update". Best to just do one at a time. I'll try and check regularly during 0900-1700 UTC - is there any overlap with your hours?

[jhgg]

Updated.