Clean up Section in BasicConfig.h:

The following changes were made:
- Removed dependency on template defined in beast detail namespace.
- Removed Section::find() method which had an obsolete interface.
- Made Section::get<>() easier to use for the common case of
  retrieving a std::string.  The revised get() method replaces old
  calls to Section::find().
- Provided a default template parameter to free function
  get<>(Section config, std::string name) so it stays similar to
  Section::get<>().

Then the rest of the code was adapted to these changes.

- Calls to Section::find() were replaced with calls to Section::get.
- Unnecessary get<std::string>() arguments were reduced to get().

These changes dug up an interesting artifact in the SHAMap unit
tests.  I'm not sure why the tests were working before, but there
was a problem with the case of a Section key.  The unit test is
fixed.

src/ripple/core/impl/Config.cpp

@@ -671,7 +671,7 @@ Config::loadFromString(std::string const& fileContents)
 
         try
         {
-            if (auto val = sec.get<std::string>("max_unknown_time"))
+            if (auto val = sec.get("max_unknown_time"))
                 MAX_UNKNOWN_TIME =
                     seconds{beast::lexicalCastThrow<std::uint32_t>(*val)};
         }
@@ -689,7 +689,7 @@ Config::loadFromString(std::string const& fileContents)
 
         try
         {
-            if (auto val = sec.get<std::string>("max_diverged_time"))
+            if (auto val = sec.get("max_diverged_time"))
                 MAX_DIVERGED_TIME =
                     seconds{beast::lexicalCastThrow<std::uint32_t>(*val)};
         }