chore: improve GetQueryMap performance.

[appleboy]

See the benchmark results:

$ go test -benchmem -run=^$ github.com/gin-gonic/gin -bench ^BenchmarkGetQueryMap
goos: darwin
goarch: amd64
pkg: github.com/gin-gonic/gin
BenchmarkGetQueryMapOld-4        1000000              1810 ns/op             880 B/op         11 allocs/op
BenchmarkGetQueryMapNew-4        3000000               404 ns/op             336 B/op          2 allocs/op
PASS
ok      github.com/gin-gonic/gin        4.442s

[codecov]

Codecov Report

Merging #1918 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1918      +/-   ##
==========================================
+ Coverage   98.75%   98.75%   +<.01%     
==========================================
  Files          38       38              
  Lines        2164     2167       +3     
==========================================
+ Hits         2137     2140       +3     
  Misses         15       15              
  Partials       12       12

Impacted Files	Coverage Δ
context.go	98.4% <100%> (+0.01%)	⬆️

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[thinkerou]

LGTM 👍