random: convert GetRand{Micros,Millis} into randrange

There are only a few call sites of these throughout the codebase, so
move the functionality into FastRandomContext, and rewrite all call sites.

This requires the callers to explicit construct FastRandomContext objects,
which do add to the verbosity, but also make potentially apparent locations
where the code can be improved by reusing a FastRandomContext object (see
further commit).

src/net_processing.cpp

@@ -1698,7 +1698,7 @@ void PeerManagerImpl::ReattemptInitialBroadcast(CScheduler& scheduler)
 
     // Schedule next run for 10-15 minutes in the future.
     // We add randomness on every cycle to avoid the possibility of P2P fingerprinting.
-    const std::chrono::milliseconds delta = 10min + GetRandMillis(5min);
+    const auto delta = 10min + FastRandomContext().randrange<std::chrono::milliseconds>(5min);
     scheduler.scheduleFromNow([&] { ReattemptInitialBroadcast(scheduler); }, delta);
 }
 
@@ -2050,7 +2050,7 @@ void PeerManagerImpl::StartScheduledTasks(CScheduler& scheduler)
     scheduler.scheduleEvery([this] { this->CheckForStaleTipAndEvictPeers(); }, std::chrono::seconds{EXTRA_PEER_CHECK_INTERVAL});
 
     // schedule next run for 10-15 minutes in the future
-    const std::chrono::milliseconds delta = 10min + GetRandMillis(5min);
+    const auto delta = 10min + FastRandomContext().randrange<std::chrono::milliseconds>(5min);
     scheduler.scheduleFromNow([&] { ReattemptInitialBroadcast(scheduler); }, delta);
 }
 
@@ -5753,7 +5753,7 @@ void PeerManagerImpl::MaybeSendFeefilter(CNode& pto, Peer& peer, std::chrono::mi
     // until scheduled broadcast, then move the broadcast to within MAX_FEEFILTER_CHANGE_DELAY.
     else if (current_time + MAX_FEEFILTER_CHANGE_DELAY < peer.m_next_send_feefilter &&
                 (currentFilter < 3 * peer.m_fee_filter_sent / 4 || currentFilter > 4 * peer.m_fee_filter_sent / 3)) {
-        peer.m_next_send_feefilter = current_time + GetRandomDuration<std::chrono::microseconds>(MAX_FEEFILTER_CHANGE_DELAY);
+        peer.m_next_send_feefilter = current_time + FastRandomContext().randrange<std::chrono::microseconds>(MAX_FEEFILTER_CHANGE_DELAY);
     }
 }