Move OperationLimiter to DnsResolver

OperationLimiter logically belongs with its only customer, DnsResolver.

I added OperationLimiter to libnetdutils for code sharing, but it turns
out netd doesn't have any need for this because it doesn't include other
threaded servers.

Change-Id: Id48cd47c6d257b63a276efb91bd3b270357c8c13