* Change DNS cache to use std::map.
Revert commit 90c259d84e, because if we use unordered_map, toString() can be inconsistent.
* Move ClientKnob::COORDINATOR_HOSTNAME_RESOLVE_DELAY to FlowKnob::HOSTNAME_RESOLVE_DELAY.
* Move Hostname to its own files.
Also, add resolve-related variables and functions in Hostname.