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/Pg.cpp

@@ -347,7 +347,7 @@ PgPool::PgPool(Section const& pgConfig, beast::Journal j) : j_(j)
 
     // The connection object must be freed using the libpq API PQfinish() call.
     pg_connection_type conn(
-        PQconnectdb(get<std::string>(pgConfig, "conninfo").c_str()),
+        PQconnectdb(get(pgConfig, "conninfo").c_str()),
         [](PGconn* conn) { PQfinish(conn); });
     if (!conn)
         Throw<std::runtime_error>("Can't create DB connection.");