Pool DoT connections

[ignoramous]

For potential latency and bandwidth savings.

refs:

• conncheck: use unix.Poll instead of syscall.Read go-sql-driver/mysql#1456
• "closing bad idle connection: unexpected read from socket" errors on MySQL 8.0.24 go-sql-driver/mysql#1392
• RFC: Disable CheckConnLiveness by default. go-sql-driver/mysql#1451 (comment)
• Pool TLS connections aren't long-lived anymore redis/go-redis#3137 (comment)
• High memory footprint per connection grpc/grpc-go#5751
• net: add mechanism to wait for readability on a TCPConn golang/go#15735

[ignoramous]

# with pooling (retrier only has one outgoing packet corresponding to one DNS query)
➜  firestack git:(n2) ✗ go clean -testcache && go test -timeout 30s -run ^TestDot$ github.com/celzero/firestack/intra/dns53 -v | grep -iE "(pool|retrier)"
retrier.go:220: D retrier: dial(SplitAuto,RetryAfterSplit,) 10.1.204.199:44998->213.188.216.9:853; strat: 1, rtt: 39ms; err? <nil>
retrier.go:357: I retrier: write: first?(true) [10.1.204.199:44998=>213.188.216.9:853] 1490; 1st write-err? <nil>
retrier.go:289: V retrier: read: [10.1.204.199:44998<=213.188.216.9:853] 576; err: <nil>
retrier.go:289: V retrier: read: [10.1.204.199:44998<=213.188.216.9:853] 2864; err: <nil>
retrier.go:289: V retrier: read: [10.1.204.199:44998<=213.188.216.9:853] 626; err: <nil>
dot.go:221: V dot: pool: (test0) put for 824634754688; ok? true
connpool.go:245: D pool: 10.1.204.199:44998=>213.188.216.9:853 readable? true; err? <nil>
connpool.go:163: D pool: 824634754688 get: empty? false, timedout? false
dot.go:239: V dot: pool: (test0) got conn from 824634754688; 1
retrier.go:289: V retrier: read: [10.1.204.199:44998<=213.188.216.9:853] 141; err: <nil>
dot.go:221: V dot: pool: (test0) put for 824634754688; ok? true
connpool.go:245: D pool: 10.1.204.199:44998=>213.188.216.9:853 readable? true; err? <nil>
connpool.go:163: D pool: 824634754688 get: empty? false, timedout? false
dot.go:239: V dot: pool: (test0) got conn from 824634754688; 2
retrier.go:289: V retrier: read: [10.1.204.199:44998<=213.188.216.9:853] 201; err: <nil>
dot.go:221: V dot: pool: (test0) put for 824634754688; ok? true
connpool.go:245: D pool: 10.1.204.199:44998=>213.188.216.9:853 readable? true; err? <nil>
connpool.go:163: D pool: 824634754688 get: empty? false, timedout? false
dot.go:239: V dot: pool: (test0) got conn from 824634754688; 3
retrier.go:289: V retrier: read: [10.1.204.199:44998<=213.188.216.9:853] 273; err: <nil>
dot.go:221: V dot: pool: (test0) put for 824634754688; ok? true

# without pooling (retrier consumes 2x bandwidth; 2 outgoing packets per DNS query)
➜  firestack git:(n2) ✗ go clean -testcache && go test -timeout 30s -run ^TestDot$ github.com/celzero/firestack/intra/dns53 -v | grep -iE "(pool|retrier)"
retrier.go:220: D retrier: dial(SplitAuto,RetryAfterSplit,) 10.1.204.199:36782->213.188.216.9:853; strat: 1, rtt: 40ms; err? <nil>
retrier.go:357: I retrier: write: first?(true) [10.1.204.199:36782=>213.188.216.9:853] 1490; 1st write-err? <nil>
retrier.go:289: V retrier: read: [10.1.204.199:36782<=213.188.216.9:853] 576; err: <nil>
retrier.go:289: V retrier: read: [10.1.204.199:36782<=213.188.216.9:853] 2865; err: <nil>
retrier.go:289: V retrier: read: [10.1.204.199:36782<=213.188.216.9:853] 542; err: <nil>
retrier.go:289: V retrier: read: [10.1.204.199:36782<=213.188.216.9:853] 84; err: <nil>
retrier.go:220: D retrier: dial(SplitAuto,RetryAfterSplit,) 10.1.204.199:36794->213.188.216.9:853; strat: 1, rtt: 43ms; err? <nil>
retrier.go:357: I retrier: write: first?(true) [10.1.204.199:36794=>213.188.216.9:853] 1761; 1st write-err? <nil>
retrier.go:289: V retrier: read: [10.1.204.199:36794<=213.188.216.9:853] 225; err: <nil>
retrier.go:289: V retrier: read: [10.1.204.199:36794<=213.188.216.9:853] 141; err: <nil>
retrier.go:220: D retrier: dial(SplitAuto,RetryAfterSplit,) 10.1.204.199:36810->213.188.216.9:853; strat: 1, rtt: 42ms; err? <nil>
retrier.go:357: I retrier: write: first?(true) [10.1.204.199:36810=>213.188.216.9:853] 1761; 1st write-err? <nil>
retrier.go:289: V retrier: read: [10.1.204.199:36810<=213.188.216.9:853] 225; err: <nil>
retrier.go:289: V retrier: read: [10.1.204.199:36810<=213.188.216.9:853] 201; err: <nil>
retrier.go:220: D retrier: dial(SplitAuto,RetryAfterSplit,) 10.1.204.199:36822->213.188.216.9:853; strat: 1, rtt: 44ms; err? <nil>
retrier.go:357: I retrier: write: first?(true) [10.1.204.199:36822=>213.188.216.9:853] 1761; 1st write-err? <nil>
retrier.go:289: V retrier: read: [10.1.204.199:36822<=213.188.216.9:853] 225; err: <nil>
retrier.go:289: V retrier: read: [10.1.204.199:36822<=213.188.216.9:853] 273; err: <nil>