reuse DomainRecord don't alway allocate new struct

[yujinqiu]

reuse DomainRecord don't alway allocate new struct, make GC happier.

[DarienRaymond]

Please provide benchmark data to support this change. Expired records is cleaned up in a timely manner (10 minutes). The case you try to optimize doesn't happen that often.

[codecov]

Codecov Report

Merging #984 into master will increase coverage by 0.19%.
The diff coverage is 44.44%.

@@            Coverage Diff             @@
##           master     #984      +/-   ##
==========================================
+ Coverage   75.47%   75.66%   +0.19%     
==========================================
  Files         206      206              
  Lines        9462     9467       +5     
==========================================
+ Hits         7141     7163      +22     
+ Misses       1653     1638      -15     
+ Partials      668      666       -2

Impacted Files	Coverage Δ
app/dns/server.go	75.64% <44.44%> (-5.19%)	⬇️
common/buf/copy.go	68.62% <0%> (-7.85%)	⬇️
common/log/logger.go	84.37% <0%> (-3.13%)	⬇️
proxy/vmess/outbound/outbound.go	79.77% <0%> (+1.12%)	⬆️
transport/internet/kcp/sending.go	88.44% <0%> (+1.33%)	⬆️
transport/internet/headers/http/http.go	81.56% <0%> (+2.12%)	⬆️
proxy/socks/client.go	75.3% <0%> (+2.46%)	⬆️
app/proxyman/mux/mux.go	59.38% <0%> (+2.62%)	⬆️
app/proxyman/outbound/handler.go	71.42% <0%> (+2.85%)	⬆️
app/proxyman/mux/session.go	85.91% <0%> (+15.49%)	⬆️

---

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 1dadd54...74f6af2. Read the comment docs.

[DarienRaymond]

Closing due to inactivity.