Update concurrent_map.go

[gua-pian]

use the std lib .

[orcaman]

@gua-pian thanks for contributing! Please see the discussion we've had here: #48

The TL:DR; is that we need to include some benchmarks in this repo (go test -bench) to make sure these type of changes do not hurt performance for the users of the package.

Eventually, in the discussion we've had on the stdlib's page here, the native implementation did seem a tad slower, but perhaps it's just the number of test iterations.

[gua-pian]

@orcaman thanks for your reply, I'll close this.