make batchConn more generic

[zjx20]

Sometimes we would like to implement a custom net.PacketConn, for example the tcpraw. This patch allows kcp-go to leverage the batching ability provided by the custom conn, if possible.

[codecov]

Codecov Report

❌ Patch coverage is 86.17021% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.29%. Comparing base (7450e91) to head (b930b19).
⚠️ Report is 324 commits behind head on master.

Files with missing lines	Patch %	Lines
batchconn_linux.go	72.22%	6 Missing and 4 partials ⚠️
readloop.go	92.10%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #200      +/-   ##
==========================================
+ Coverage   82.47%   83.29%   +0.82%     
==========================================
  Files          12       11       -1     
  Lines        1889     1910      +21     
==========================================
+ Hits         1558     1591      +33     
+ Misses        260      250      -10     
+ Partials       71       69       -2     

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