Reduce allocations in (*routeRegexp).getURLQuery

[klauspost]

Summary of Changes

A production server is seeing a significant amount of allocations in (*routeRegexp).getURLQuery

Since it is only interested in a single value and only the first value we create a specialized function for that.

Comparing a few parameter parsing scenarios:

Benchmark_findQueryKey/0-8 	 7184014	       168 ns/op	       0 B/op	       0 allocs/op
Benchmark_findQueryKey/1-8 	 5307873	       227 ns/op	      48 B/op	       3 allocs/op
Benchmark_findQueryKey/2-8 	 1560836	       770 ns/op	     483 B/op	      10 allocs/op
Benchmark_findQueryKey/3-8 	 1296200	       931 ns/op	     559 B/op	      11 allocs/op
Benchmark_findQueryKey/4-8 	  666502	      1769 ns/op	       3 B/op	       1 allocs/op

Benchmark_findQueryKeyGoLib/0-8 	 1740973	       690 ns/op	     864 B/op	       8 allocs/op
Benchmark_findQueryKeyGoLib/1-8 	 3029618	       393 ns/op	     432 B/op	       4 allocs/op
Benchmark_findQueryKeyGoLib/2-8 	  461427	      2511 ns/op	    1542 B/op	      24 allocs/op
Benchmark_findQueryKeyGoLib/3-8 	  324252	      3804 ns/op	    1984 B/op	      28 allocs/op
Benchmark_findQueryKeyGoLib/4-8 	   69348	     14928 ns/op	   12716 B/op	     130 allocs/op

Test included.

[elithrar]

This is great - thanks for catching this and improving (w/ tests & benchmarks!).

[klauspost]

Thanks! Looking forward to the next release :D