Support session per connection mode (like GoQuiet) when NumConn = 0.

[notsure2]

This adds a "goquiet" mode to Cloak where every TCP connection is handled in a separate session. This maybe useful for people on unstable connections where the DPI/GFW cuts long time idle HTTPS connections (by dropping packets without tcp reset) and Cloak takes a very long time or doesn't recover until it's restarted. To use this mode, just set NumConn=0 in the config.

[codecov]

Codecov Report

Merging #122 into master will decrease coverage by 0.83%.
The diff coverage is 34.88%.

@@            Coverage Diff             @@
##           master     #122      +/-   ##
==========================================
- Coverage   63.91%   63.07%   -0.84%     
==========================================
  Files          38       38              
  Lines        2544     2573      +29     
==========================================
- Hits         1626     1623       -3     
- Misses        771      795      +24     
- Partials      147      155       +8     

Impacted Files	Coverage Δ
cmd/ck-client/ck-client.go	0.00% <0.00%> (ø)
internal/client/state.go	50.44% <0.00%> (ø)
internal/client/piper.go	41.83% <35.48%> (-8.17%)	⬇️
internal/client/connector.go	68.62% <57.14%> (-3.72%)	⬇️
internal/server/TLS.go	59.61% <0.00%> (-7.70%)	⬇️
internal/multiplex/bufferedPipe.go	89.87% <0.00%> (-3.80%)	⬇️
internal/multiplex/obfs.go	81.05% <0.00%> (-2.11%)	⬇️
internal/multiplex/session.go	82.24% <0.00%> (-1.19%)	⬇️
internal/server/websocket.go	40.00% <0.00%> (+6.00%)	⬆️

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[cbeuw]

Thanks!