Fix deprecated behaviour in gloo/rendezvous/store.h

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: D54158184

fbshipit-source-id: f3b98790d6592ac21e6d002b2d0d914bc331735d

Author: r-barnes

gloo/rendezvous/store.cc

@@ -11,10 +11,6 @@
 namespace gloo {
 namespace rendezvous {
 
-constexpr std::chrono::milliseconds Store::kDefaultTimeout;
-
-// Have to provide implementation for pure virtual destructor.
-Store::~Store() {}
 
 } // namespace rendezvous
 } // namespace gloo

gloo/rendezvous/store.h

@@ -27,7 +27,7 @@ class Store: public IStore {
   static constexpr std::chrono::milliseconds kDefaultTimeout =
       std::chrono::seconds(30);
 
-  virtual ~Store();
+  virtual ~Store() = default;
 
   virtual void set(const std::string& key, const std::vector<char>& data) = 0;