Micro optimization in Tag Cache

[deankarn]

Removed defer from the simple Tag Cache lock & unlock and reduced
execution time for even single field validation by almost 100ns
from 254ns/op to 163ns/op .. Not badd for such a simple modification
and this effect ripples throughout struct validation.

$ go test -cpu=4 -bench=. -benchmem=true
PASS
BenchmarkFieldSuccess-4                             10000000 163 ns/op         0 B/op          0 allocs/op
BenchmarkFieldFailure-4                              2000000 673 ns/op       400 B/op          4 allocs/op
BenchmarkFieldDiveSuccess-4                           500000 3019 ns/op      480 B/op         27 allocs/op
BenchmarkFieldDiveFailure-4                           500000 3553 ns/op      880 B/op         31 allocs/op
BenchmarkFieldCustomTypeSuccess-4                    5000000 347 ns/op        32 B/op          2 allocs/op
BenchmarkFieldCustomTypeFailure-4                    2000000 645 ns/op       400 B/op          4 allocs/op
BenchmarkFieldOrTagSuccess-4                         1000000 1177 ns/op       16 B/op          1 allocs/op
BenchmarkFieldOrTagFailure-4                         1000000 1093 ns/op      432 B/op          6 allocs/op
BenchmarkStructLevelValidationSuccess-4              2000000 702 ns/op       160 B/op          6 allocs/op
BenchmarkStructLevelValidationFailure-4              1000000 1279 ns/op      592 B/op         11 allocs/op
BenchmarkStructSimpleCustomTypeSuccess-4             1000000 1010 ns/op       80 B/op          5 allocs/op
BenchmarkStructSimpleCustomTypeFailure-4             1000000 1544 ns/op      624 B/op         11 allocs/op
BenchmarkStructPartialSuccess-4                      1000000 1249 ns/op      400 B/op         11 allocs/op
BenchmarkStructPartialFailure-4                      1000000 1797 ns/op      816 B/op         16 allocs/op
BenchmarkStructExceptSuccess-4                       2000000 927 ns/op       368 B/op          9 allocs/op
BenchmarkStructExceptFailure-4                       1000000 1259 ns/op      400 B/op         11 allocs/op
BenchmarkStructSimpleCrossFieldSuccess-4             1000000 1076 ns/op      128 B/op          6 allocs/op
BenchmarkStructSimpleCrossFieldFailure-4             1000000 1623 ns/op      560 B/op         11 allocs/op
BenchmarkStructSimpleCrossStructCrossFieldSuccess-4  1000000 1582 ns/op      176 B/op          9 allocs/op
BenchmarkStructSimpleCrossStructCrossFieldFailure-4  1000000 2139 ns/op      608 B/op         14 allocs/op
BenchmarkStructSimpleSuccess-4                       1000000 1040 ns/op       48 B/op          3 allocs/op
BenchmarkStructSimpleFailure-4                       1000000 1683 ns/op      624 B/op         11 allocs/op
BenchmarkStructSimpleSuccessParallel-4               5000000 356 ns/op        48 B/op          3 allocs/op
BenchmarkStructSimpleFailureParallel-4               2000000 831 ns/op       624 B/op         11 allocs/op
BenchmarkStructComplexSuccess-4                       200000 6738 ns/op      512 B/op         30 allocs/op
BenchmarkStructComplexFailure-4                       200000 11387 ns/op    3415 B/op         72 allocs/op
BenchmarkStructComplexSuccessParallel-4               500000 2330 ns/op      512 B/op         30 allocs/op
BenchmarkStructComplexFailureParallel-4               300000 4857 ns/op     3416 B/op         72 allocs/op

$ go test -cpu=4 -bench=. -benchmem=true
PASS
BenchmarkFieldSuccess-4                              5000000 254 ns/op         0 B/op          0 allocs/op
BenchmarkFieldFailure-4                              2000000 779 ns/op       400 B/op          4 allocs/op
BenchmarkFieldDiveSuccess-4                           500000 3451 ns/op      480 B/op         27 allocs/op
BenchmarkFieldDiveFailure-4                           300000 3954 ns/op      880 B/op         31 allocs/op
BenchmarkFieldCustomTypeSuccess-4                    3000000 451 ns/op        32 B/op          2 allocs/op
BenchmarkFieldCustomTypeFailure-4                    2000000 751 ns/op       400 B/op          4 allocs/op
BenchmarkFieldOrTagSuccess-4                         1000000 1312 ns/op       16 B/op          1 allocs/op
BenchmarkFieldOrTagFailure-4                         1000000 1206 ns/op      432 B/op          6 allocs/op
BenchmarkStructLevelValidationSuccess-4              2000000 829 ns/op       160 B/op          6 allocs/op
BenchmarkStructLevelValidationFailure-4              1000000 1403 ns/op      592 B/op         11 allocs/op
BenchmarkStructSimpleCustomTypeSuccess-4             1000000 1238 ns/op       80 B/op          5 allocs/op
BenchmarkStructSimpleCustomTypeFailure-4             1000000 1771 ns/op      624 B/op         11 allocs/op
BenchmarkStructPartialSuccess-4                      1000000 1399 ns/op      400 B/op         11 allocs/op
BenchmarkStructPartialFailure-4                      1000000 1926 ns/op      816 B/op         16 allocs/op
BenchmarkStructExceptSuccess-4                       2000000 930 ns/op       368 B/op          9 allocs/op
BenchmarkStructExceptFailure-4                       1000000 1381 ns/op      400 B/op         11 allocs/op
BenchmarkStructSimpleCrossFieldSuccess-4             1000000 1253 ns/op      128 B/op          6 allocs/op
BenchmarkStructSimpleCrossFieldFailure-4             1000000 1849 ns/op      560 B/op         11 allocs/op
BenchmarkStructSimpleCrossStructCrossFieldSuccess-4  1000000 1902 ns/op      176 B/op          9 allocs/op
BenchmarkStructSimpleCrossStructCrossFieldFailure-4   500000 2453 ns/op      608 B/op         14 allocs/op
BenchmarkStructSimpleSuccess-4                       1000000 1182 ns/op       48 B/op          3 allocs/op
BenchmarkStructSimpleFailure-4                       1000000 1842 ns/op      624 B/op         11 allocs/op
BenchmarkStructSimpleSuccessParallel-4               5000000 342 ns/op        48 B/op          3 allocs/op
BenchmarkStructSimpleFailureParallel-4               2000000 665 ns/op       624 B/op         11 allocs/op
BenchmarkStructComplexSuccess-4                       200000 7924 ns/op      512 B/op         30 allocs/op
BenchmarkStructComplexFailure-4                       100000 12605 ns/op    3415 B/op         72 allocs/op
BenchmarkStructComplexSuccessParallel-4              1000000 2421 ns/op      512 B/op         30 allocs/op
BenchmarkStructComplexFailureParallel-4               300000 4234 ns/op     3416 B/op         72 allocs/op