add benchmarks

[ebuckley]

I wanted to explore this repo a bit by writing a benchmark test.

IS the Benchmark1point6mill/1.6_million_locks_unlocks_(map_version)-16 implementation representative of the current way we do locking? it looks like it is a bit faster in the naive test of making a map of locks with 1.6 million entries.

goos: linux
goarch: amd64
pkg: github.com/starboard-nz/lockunique
cpu: 12th Gen Intel(R) Core(TM) i7-1260P
Benchmark1point6mill/1.6_million_locks-16         	
1000000000	         0.6862 ns/op
Benchmark1point6mill/1.6_million_locks_unlocks_(map_version)-16         	1000000000	         0.3093 ns/op
PASS
ok  	github.com/starboard-nz/lockunique	44.513s