Configure global thread pool; use atomics instead of mutex

[jvkersch]

My apologies for not opening an issue first; this PR arose out of some experimentation and I thought it would be easier to describe the issue with the code to refer to.

This PR does two things:

• Configure the global thread pool (.build_global()) rather than a local one (.build()) to ensure that subsequent calls to Rayon parallelization routines use the correct number of threads. Without this change, the --threads argument had essentially no effect for me (rayon would select an optimal number of threads, ignoring the specified value).
• Swap out the use of mutexes for atomic integers. After configuring the global thread pool I noticed there was almost no parallellization, due to the frequent locking and unlocking of the number of reads processed/output. Using atomic integers removes this bottleneck.

This is somewhat of an experimental approach, but I see the following timings (macOS, m1) while filtering a 400 MB fastq file with differing numbers of threads (time target/debug/chopper -l 200 -q 7 --threads $N < in.fastq > out.fastq). Specifically, I looked at 1, 2, 4, 8 threads and recorded the following timings:

• Code in master: 8.97s, 8.96s, 8.92s, 8.94s
• With .build_global() and using mutexes: 9.13s, 9.52s, 9.17s, 8.90s
• With .build_global() and atomics (this PR): 9.32s, 5.01s, 3.53s, 3.61s

[wdecoster]

Awesome, thank you, I will follow up soon, after my holidays.

[wdecoster]

Looks good to me :) thanks for the improvements!