Call Clock::now only once in writeConnectionDataToSocket

Summary:
The time between iterations is not significant, so we can just call `Clock::now` once in the beginning and reuse the same value.

I ran a canary with some counters to get an idea of the amount of time between the start of the first iteration and the end of the last iteration (see D57510979), and:
* Edge p100: 1500 us
* olb p100: 1900 us
* Edge p99: 413 us
* olb p99: 396 us

The wins we're seeing are 0.13% relative CPU.

Reviewed By: kvtsoy

Differential Revision: D57594650

fbshipit-source-id: 9d0f827564179745cd83eb6ca211df68d3f23f8b

quic/api/QuicTransportFunctions.cpp

@@ -1529,6 +1529,8 @@ WriteQuicDataResult writeConnectionDataToSocket(
     }
   };
 
+  quic::TimePoint sentTime = Clock::now();
+
   while (scheduler.hasData() && ioBufBatch.getPktSent() < packetLimit &&
          ((ioBufBatch.getPktSent() < batchSize) ||
           writeLoopTimeLimit(writeLoopBeginTime, connection))) {
@@ -1581,7 +1583,7 @@ WriteQuicDataResult writeConnectionDataToSocket(
         connection,
         std::move(result->packetEvent),
         std::move(result->packet->packet),
-        Clock::now(),
+        sentTime,
         folly::to<uint32_t>(ret.encodedSize),
         folly::to<uint32_t>(ret.encodedBodySize),
         false /* isDSRPacket */);