Optimize SASL auth in sqlx-postgres

[mirek26]

Don't recreate the HMAC, just reset. That feature exists for a reason :) This makes postgres sasl auth 2-3ms faster (or 40ms in debug mode).

Benchmark results (with +nightly #![feature(test)] extern crate test;) on M1 Pro:

before:

test connection::sasl::bench_sasl_hi                                        ... bench:   4,004,666 ns/iter (+/- 73,042)

after

test connection::sasl::bench_sasl_hi                                        ... bench:   2,065,200 ns/iter (+/- 15,375)