Reduce Beast public interface and eliminate unused code:

Beast includes a lot of code for encapsulating cross-platform differences
which are not used or needed by rippled. Additionally, a lot of that code
implements functionality that is available from the standard library.

This moves away from custom implementations of features that the standard
library provides and reduces the number of platform-specific interfaces
andfeatures that Beast makes available.

Highlights include:
* Use std:: instead of beast implementations when possible
* Reduce the use of beast::String in public interfaces
* Remove Windows-specific COM and Registry code
* Reduce the public interface of beast::File
* Reduce the public interface of beast::SystemStats
* Remove unused sysctl/getsysinfo functions
* Remove beast::Logger

beast/threads/impl/Thread.cpp

@@ -268,23 +268,6 @@ void Thread::notify() const
     defaultEvent.signal();
 }
 
-//==============================================================================
-
-// This is here so we dont have circular includes
-//
-void SpinLock::enter() const noexcept
-{
-    if (! tryEnter())
-    {
-        for (int i = 20; --i >= 0;)
-            if (tryEnter())
-                return;
-
-        while (! tryEnter())
-            Thread::yield();
-    }
-}
-
 }
 
 //------------------------------------------------------------------------------
@@ -429,11 +412,6 @@ void Thread::sleep (const int millisecs)
     }
 }
 
-void Thread::yield()
-{
-    Sleep (0);
-}
-
 }
 
 //------------------------------------------------------------------------------
@@ -566,11 +544,6 @@ Thread::ThreadID Thread::getCurrentThreadId()
     return (ThreadID) pthread_self();
 }
 
-void Thread::yield()
-{
-    sched_yield();
-}
-
 //==============================================================================
 /* Remove this macro if you're having problems compiling the cpu affinity
    calls (the API for these has changed about quite a bit in various Linux