Make the fp locker much simpler.

Benchmark:
$ go test -bench 'Fingerprint' -test.run 'Fingerprint' -test.cpu=1,2,4

OLD
BenchmarkFingerprintLockerParallel        500000              3618 ns/op
BenchmarkFingerprintLockerParallel-2      100000             12257 ns/op
BenchmarkFingerprintLockerParallel-4      500000             10164 ns/op
BenchmarkFingerprintLockerSerial        10000000               283 ns/op
BenchmarkFingerprintLockerSerial-2      10000000               284 ns/op
BenchmarkFingerprintLockerSerial-4      10000000               288 ns/op

NEW
BenchmarkFingerprintLockerParallel       1000000              1018 ns/op
BenchmarkFingerprintLockerParallel-2     1000000              1164 ns/op
BenchmarkFingerprintLockerParallel-4     2000000               910 ns/op
BenchmarkFingerprintLockerSerial        50000000                56.0 ns/op
BenchmarkFingerprintLockerSerial-2      50000000                47.9 ns/op
BenchmarkFingerprintLockerSerial-4      50000000                54.5 ns/op

Change-Id: I338c4cff34bde3b261e2dcf8172756b9f6f577db