perf: improved func ValidateNodeForTxn node id parsing

[denis-gudim]

Enhanced permission ValidateNodeForTxn function contains repeating parsing of enode identifier from NodeInfo.Url field. This fix stores parsed enode id value to cached NodeInfo object and reuse stored value on each ValidateNodeForTxn function call.

In our EOA transactions performance test with enabled enhanced permission, this fix decrease CPU usage on ~10% in collected pprof report.

Benchmark source:

goos: linux
goarch: amd64
pkg: github.com/ethereum/go-ethereum/permission/core
cpu: Intel(R) Xeon(R) CPU E5-2696 v3 @ 2.30GHz
BenchmarkValidateNodeForTxn-8             100000             17177 ns/op           11911 B/op        181 allocs/op
BenchmarkValidateNodeForTxn-8             100000             16033 ns/op           11914 B/op        181 allocs/op
BenchmarkValidateNodeForTxn-8             100000             16371 ns/op           11886 B/op        181 allocs/op
BenchmarkValidateNodeForTxn-8             100000             16227 ns/op           11915 B/op        181 allocs/op
BenchmarkValidateNodeForTxn-8             100000             15923 ns/op           11870 B/op        181 allocs/op
PASS
ok      github.com/ethereum/go-ethereum/permission/core 8.204s

Benchmark fix:

goos: linux
goarch: amd64
pkg: github.com/ethereum/go-ethereum/permission/core
cpu: Intel(R) Xeon(R) CPU E5-2696 v3 @ 2.30GHz
BenchmarkValidateNodeForTxn-8             100000              1104 ns/op              40 B/op          3 allocs/op
BenchmarkValidateNodeForTxn-8             100000              1750 ns/op              40 B/op          3 allocs/op
BenchmarkValidateNodeForTxn-8             100000              1584 ns/op              41 B/op          3 allocs/op
BenchmarkValidateNodeForTxn-8             100000              1467 ns/op              41 B/op          3 allocs/op
BenchmarkValidateNodeForTxn-8             100000              1631 ns/op              42 B/op          3 allocs/op
PASS
ok      github.com/ethereum/go-ethereum/permission/core 0.789s

[baptiste-b-pegasys]

LGTM, I raise few small remarks as I don't entirely master the subject with errors handling and go types