Make the locking slightly smarter in Evict and add benchmark function.

Seems a bit faster (there was some variance while performing these
benchmarks)

Master:
BenchmarkCache-2   	 1000000	      2317 ns/op	       0 B/op	       0 allocs/op
BenchmarkCache-2   	 1000000	      2032 ns/op	       0 B/op	       0 allocs/op

This branch:
BenchmarkCache-2   	 1000000	      1806 ns/op	       0 B/op	       0 allocs/op
BenchmarkCache-2   	 1000000	      1809 ns/op	       0 B/op	       0 allocs/op