Simplify sockopts

[9seconds]

Please take a look #208, #220 and #229

From early days mtg was not using TCP keepalive in a favor of its own "connection aborter" and allowing to tune buffer size. This was made with a purpose: I didn't find TCP keepalive to work as I wanted and we all spent so many time to find out the best buffer size so I've even made it configurable. Right now both of them look redundand for me and moreover, complexifies everything a lot. They are hackish.

This PR is an attempt to start wiping them out and I hope, this will also help us to achieve the better performance. I'm going to test it out in various conditions (fortunately or not, I'm going to connect to many different networks in forecoming weeks) to see how it performs and behaves. If I won't find any problem, I'll merge.

Any assistance or testing is also appreciated.

[codecov-commenter]

Codecov Report

Merging #230 (e7416bc) into master (bae5407) will decrease coverage by 1.72%.
The diff coverage is 64.49%.

@@            Coverage Diff             @@
##           master     #230      +/-   ##
==========================================
- Coverage   84.20%   82.47%   -1.73%     
==========================================
  Files          68       69       +1     
  Lines        1868     1929      +61     
==========================================
+ Hits         1573     1591      +18     
- Misses        210      237      +27     
- Partials       85      101      +16     

Impacted Files	Coverage Δ
internal/config/config.go	36.36% <ø> (ø)
internal/config/parse.go	84.61% <ø> (ø)
internal/utils/net_listener.go	0.00% <0.00%> (ø)
mtglib/conns.go	100.00% <ø> (ø)
mtglib/internal/faketls/conn.go	93.33% <ø> (ø)
mtglib/internal/obfuscated2/conn.go	81.81% <ø> (ø)
mtglib/proxy_opts.go	88.88% <ø> (+2.22%)	⬆️
mtglib/stream_context.go	91.30% <ø> (ø)
network/circuit_breaker.go	76.92% <ø> (-7.70%)	⬇️
network/load_balanced_socks5.go	100.00% <ø> (ø)
... and 20 more

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[EkkoG]

I'm trying to test this PR with daily usage.

[EkkoG]

With this PR, the performance is much better now.

Legacy

mtg is slower than official MTProxy about 20%, official version 320ms, mtg 390 ms

Download speed

with the same server and same video resource.

official version 3.0M/s, mtg 2.5M/s, also about 20% slower.

[9seconds]

Thanks. Seems we are on the right track.

[9seconds]

@EkkoG i'm going to tryout a couple of other options. I'm curious about your testing methodology, would you mind to share how you test a throughput?

[9seconds]

Yet another big update. Seems I've made a couple of other improvements, that improved performance a lot. At least, first results are encouraging.

[database64128]

1. There's no need to set TCP keepalive in mtg, because Go already sets it by default with a 15 second interval.
2. Calling net.TCPConn.CloseRead(), or the shutdown(SHUT_RD) syscall on a TCP socket doesn't actually do anything, other than making subsequent read(2) calls fail, which shouldn't happen anyway because there won't be any subsequent read(2) calls. Calling net.TCPConn.CloseWrite() or shutdown(SHUT_WR) alone is enough, which sends FIN to terminate data transfer of this direction.