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
```
Mariano Sosto
Klaus Post