Fix gomap datarace

[tdakkota]

delete is not thread safe and must be synchronized

[philippgille]

Hi @tdakkota, thank you so much for the 4 PRs!

I'm currently short on time, but this one is really important and it's just a small change, so I had a look. Totally makes sense, not sure how I missed that! It's great that you already changed the target branch to develop, so despite the Consul tests failing (https://travis-ci.org/philippgille/gokv/jobs/647844727#L2464) I'll merge this right now and have a look at the Consul tests as soon as possible, maybe this weekend.

I'll also have a look at the other PRs as soon as possible. Thanks a lot already, it's great to know others like the idea to have many implementations for a simple interface as well!

[tdakkota]

Hi @philippgille!
When I changed

https://github.com/philippgille/gokv/blob/master/consul/consul_test.go#L69

goroutineCount := 1000

to

goroutineCount := 10

it passed.

I think it is Consul bug