pfBlockerNG: use asynchronous I/O

[andrebrait]

• Delegate file and DB operations to a ThreadPoolExecutor
• Reduce the minimum latency when blacklisting from ~4ms to ~0ms

Before (repeated the call numerous times, always had around 4ms delay, domain is blacklisted)

[2.7.0-RELEASE][[email protected]]/root: dig AAAA static.prototypes.ru

; <<>> DiG 9.18.14 <<>> AAAA static.prototypes.ru
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 26860
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1432
;; QUESTION SECTION:
;static.prototypes.ru.          IN      AAAA

;; ANSWER SECTION:
static.prototypes.ru.   60      IN      AAAA    ::10.10.10.1

;; Query time: 4 msec
;; SERVER: 127.0.0.1#53(127.0.0.1) (UDP)
;; WHEN: Mon Oct 09 11:25:32 CEST 2023
;; MSG SIZE  rcvd: 77

After (repeated the call numerous times, always had around 1ms delay, all logging is working, domain is blacklisted):

[2.7.0-RELEASE][[email protected]]/root: dig AAAA static.prototypes.ru

; <<>> DiG 9.18.14 <<>> AAAA static.prototypes.ru
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 9209
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1432
;; QUESTION SECTION:
;static.prototypes.ru.          IN      AAAA

;; ANSWER SECTION:
static.prototypes.ru.   60      IN      AAAA    ::10.10.10.1

;; Query time: 0 msec
;; SERVER: 127.0.0.1#53(127.0.0.1) (UDP)
;; WHEN: Mon Oct 09 11:26:53 CEST 2023
;; MSG SIZE  rcvd: 77

[sarog]

@andrebrait This is nice. You need to request a review from @BBcan177 to get his attention so he can (hopefully) accept the changes. 😀

[andrebrait]

I'm already in contact with them :)

In fact, this MR was superseded already. I'm leaving it open for documentation/historical purposes, pretty much.

The goal is to have a big one that will encompass all the functionality we want to have in there.

[andrebrait]

Superseded by #1343