[CSL-2346] NTP.Client
[CSL-2346] NTP.Client

* Added comments
* `ncStatus` is updated with `NtpSyncUnavailable` if none of
 responses arrived.

input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype