Fix race condition obtaining an authorization key in meetup/client.go

[fredlawl]

Locks are effective at guarding access to resource read/writes, but the problem here is that the code does not currently consider a HTTP request to refresh a token a protected resource. We can enter into a race if a two calls to the client happen at the same time where caller A makes a request to refresh the token at the same time caller B makes a call to refresh the token. If caller A finishes first, then that token is now invalid. Caller B will have a valid token at that point, or could fail the request.

I don't have a good solution at the top of my head other than to lock everything up, and make sure the request timeouts at roughly a second or less to not hold up any threads accessing the next token too much.

Another idea is to leverage effective-go's channels and work out a solution with those. At the present time, this race is very tricky to trigger (requires much-greater scale to really hit it), so submitting an issue here as a reminder to take care of it.