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/test/shamap/common.h

@@ -57,7 +57,7 @@ public:
     {
         Section testSection;
         testSection.set("type", "memory");
-        testSection.set("Path", "SHAMap_test");
+        testSection.set("path", "SHAMap_test");
         db_ = NodeStore::Manager::instance().make_Database(
             megabytes(4), scheduler_, 1, testSection, j);
     }