fix MSVC C5219 conversion warnings in EventBase

Summary:
Fixes warnings like the following:

```
folly\io\async\EventBase.cpp(725): warning C5219: implicit conversion from '_Rep' to 'double', possible loss of data
        with
        [
            _Rep=__int64
        ]
folly\io\async\EventBase.cpp(738): warning C5219: implicit conversion from '_Rep' to 'double', possible loss of data
        with
        [
            _Rep=__int64
        ]
folly\io\async\EventBase.cpp(740): warning C5219: implicit conversion from 'size_t' to 'double', possible loss of data
```

Reviewed By: luciang

Differential Revision: D31330566

fbshipit-source-id: 86a2cd10fdad53dce9c0ee0704b26bb1bce2bf18

folly/io/async/EventBase.cpp

@@ -722,7 +722,7 @@ void EventBase::initNotificationQueue() {
 
 void EventBase::SmoothLoopTime::setTimeInterval(
     std::chrono::microseconds timeInterval) {
-  expCoeff_ = -1.0 / timeInterval.count();
+  expCoeff_ = -1.0 / static_cast<double>(timeInterval.count());
   VLOG(11) << "expCoeff_ " << expCoeff_ << " " << __PRETTY_FUNCTION__;
 }
 
@@ -735,9 +735,10 @@ void EventBase::SmoothLoopTime::addSample(
   if ((buffer_time_ + total) > buffer_interval_ && buffer_cnt_ > 0) {
     // See https://en.wikipedia.org/wiki/Exponential_smoothing for
     // more info on this calculation.
-    double coeff = exp(buffer_time_.count() * expCoeff_);
-    value_ =
-        value_ * coeff + (1.0 - coeff) * (busy_buffer_.count() / buffer_cnt_);
+    double coeff = exp(static_cast<double>(buffer_time_.count()) * expCoeff_);
+    value_ = value_ * coeff +
+        (1.0 - coeff) *
+            (static_cast<double>(busy_buffer_.count()) / buffer_cnt_);
     buffer_time_ = std::chrono::microseconds{0};
     busy_buffer_ = std::chrono::microseconds{0};
     buffer_cnt_ = 0;

folly/io/async/EventBase.h

@@ -736,7 +736,8 @@ class EventBase : public TimeoutManager,
   class SmoothLoopTime {
    public:
     explicit SmoothLoopTime(std::chrono::microseconds timeInterval)
-        : expCoeff_(-1.0 / timeInterval.count()), value_(0.0) {
+        : expCoeff_(-1.0 / static_cast<double>(timeInterval.count())),
+          value_(0.0) {
       VLOG(11) << "expCoeff_ " << expCoeff_ << " " << __PRETTY_FUNCTION__;
     }
 
@@ -749,8 +750,9 @@ class EventBase : public TimeoutManager,
     double get() const {
       // Add the outstanding buffered times linearly, to avoid
       // expensive exponentiation
-      auto lcoeff = buffer_time_.count() * -expCoeff_;
-      return value_ * (1.0 - lcoeff) + lcoeff * busy_buffer_.count();
+      auto lcoeff = static_cast<double>(buffer_time_.count()) * -expCoeff_;
+      return value_ * (1.0 - lcoeff) +
+          lcoeff * static_cast<double>(busy_buffer_.count());
     }
 
     void dampen(double factor) { value_ *= factor; }