Fix deprecated behaviour in cachelib/common/PercentileStats.cpp

Summary:
Future C++ standards and compiler upgrades will eliminate deprecated behaviour. `-Wdeprecated` identifies this behaviour and has found some in this code!

Some examples.

**Dynamic exceptions**
```
error: dynamic exception specifications are deprecated [-Werror,-Wdeprecated-dynamic-exception-spec]
```
`throw(...)` has been deprecated since C++11 and removed in C++17. In most cases we can just use `noexcept` in the rest, we can remove this.

**Implicit copy constructors**
```
error: definition of implicit copy constructor for 'XXX' is deprecated because it has a user-declared destructor [-Werror,-Wdeprecated-copy-with-dtor]
```
If you define a destructor, you need to explicitly define a copy constructor.

**Out-ofline constexpr static**
```
error: out-of-line definition of constexpr static data member is redundant in C++17 and is deprecated [-Werror,-Wdeprecated]
```
This can be simplified:
```
class MyClass {
    static constexpr my_const = 3;
};

static constexpr MyClass::my_const; // <- No longer needed!
```

Reviewed By: meyering

Differential Revision: D54158205

fbshipit-source-id: c6909e9542bdd36a096768013e791b4763975b6e

Author: r-barnes

cachelib/common/PercentileStats.cpp

@@ -19,10 +19,9 @@
 namespace facebook {
 namespace cachelib {
 namespace util {
-const std::array<double, 14> PercentileStats::kQuantiles{
+constexpr std::array<double, 14> PercentileStats::kQuantiles{
     0,    0.05, 0.1,   0.25,   0.5,     0.75,     0.9,
     0.95, 0.99, 0.999, 0.9999, 0.99999, 0.999999, 1.0};
-constexpr int PercentileStats::kDefaultWindowSize;
 
 PercentileStats::Estimates PercentileStats::estimate() {
   estimator_.flush();