opt: eliminate the fd->connection map when handling I/O events

[panjf2000]

By default, gnet utilizes the standard package golang.org/x/sys/unix to implement pollers with epoll or kqueue, on the other hand, when users run go build with build tags poll_opt, like this: go build -tags=poll_opt, gnet will switch to the optimized implementation by invoking the system calls of epoll or kqueue directly, which eliminates the HASH MAP of fd->conn, it ought to achieve a higher performance with this optimization.

[codecov]

Codecov Report

Merging #230 (5157885) into master (5548133) will decrease coverage by 1.81%.
The diff coverage is 71.28%.

@@            Coverage Diff             @@
##           master     #230      +/-   ##
==========================================
- Coverage   85.08%   83.27%   -1.82%     
==========================================
  Files          18       19       +1     
  Lines        1415     1477      +62     
==========================================
+ Hits         1204     1230      +26     
- Misses        158      188      +30     
- Partials       53       59       +6     

Flag	Coverage Δ
unittests	83.27% <71.28%> (-1.82%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
connection_bsd.go	0.00% <0.00%> (ø)
connection_linux.go	0.00% <0.00%> (ø)
acceptor_bsd.go	48.48% <48.48%> (ø)
eventloop_unix.go	72.25% <50.00%> (-1.66%)	⬇️
acceptor_linux.go	48.48% <63.15%> (ø)
server_unix.go	88.67% <83.33%> (ø)
reactor_default_linux.go	83.67% <83.67%> (ø)
reactor_default_bsd.go	92.72% <92.72%> (ø)
connection_unix.go	75.93% <100.00%> (+0.36%)	⬆️
gnet.go	73.17% <100.00%> (ø)
... and 7 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5548133...5157885. Read the comment docs.