Improve plugin loader

[nathanielhourt]

Replaces #1424

This PR adds the ability for client code of the application class to request plugins be loaded automatically when it registers them. I don't know if upstream bitshares will want this or not, but here's a PR just in case!

This PR is mostly a clean-up/correctness fix (the old conflict detection between the account_history and elasticsearch plugins was clumsy and would false-positive if either plugin showed up twice in the list), but it does add the ability to mark plugins to be loaded automatically when they are registered.

Please note that this PR does remove the hard-coded list of "wanted" plugins, as this shortcut is annoying to third party code, like that of Follow My Vote, which may not want to be cluttered up by having those plugins linked in. This may cause mild turbulence to existing environments which depend on this shortcut. #sorrynotsorry =) This turbulence can be avoided by calling register_plugin(true) for the desired auto-load plugins, however, doing so will prevent disabling these plugins from the config.

[cogutvalera]

there is error in cli_test:

/home/travis/build/bitshares/bitshares-core/tests/cli/main.cpp(552): error in "account_history_pagination": check 201 == history.size() failed [201 != 0]

This error is caused by if (options.count("plugins")), why did you remove else branch ? what will be if there are no options.count("plugins") ? IMHO you should add in else branch next snippet of code as was before:

      plugins.emplace("witness");
      plugins.emplace("account_history");
      plugins.emplace("market_history");
      plugins.emplace("grouped_orders");

and after that (I mean if/else statement) we should use next snippet of code:

   FC_ASSERT(!(plugins.count("account_history") && plugins.count("elasticsearch")),
      "Plugin conflict: Cannot load both account_history plugin and elasticsearch plugin");
   std::for_each(plugins.begin(), plugins.end(), [this](const string& plug) mutable {
   if (!plug.empty())
      enable_plugin(plug);
   });

Now the problem is in cli_test that it cannot use account_history plugin because it wasn't loaded.

[cogutvalera]

final code looks something like next (libraries/app/application.cpp):

   std::set<string> plugins;
   if (options.count("plugins")) {
      boost::split(plugins, options.at("plugins").as<std::string>(), [](char c){return c == ' ';});
   } else {
      plugins.emplace("witness");
      plugins.emplace("account_history");
      plugins.emplace("market_history");
      plugins.emplace("grouped_orders");
   }

   FC_ASSERT(!(plugins.count("account_history") && plugins.count("elasticsearch")),
      "Plugin conflict: Cannot load both account_history plugin and elasticsearch plugin");

   std::for_each(plugins.begin(), plugins.end(), [this](const string& plug) mutable {
      if (!plug.empty())
         enable_plugin(plug);
   });

[cogutvalera]

with my final code cli_test will pass successfully without any errors detected

[pmconrad]

Thanks!

IMO, with our current architecture, we don't need this functionality, as there are no mandatory plugins. All mandatory stuff is inside the core.

We should not remove the default selection of plugins, because we don't want to break existing installations that rely on defaults.

The conflict detection code looks better than before.

[oxarbitrage]

closing this one as the best candidate is #1437

will be merged after getting 1 approval.