get_accounts_by_authorizers multi-threaded support

[heifner]

/v1/chain/get_accounts_by_authorizers was designed to support multi-threaded calls. However, it was not hooked up to the http thread handling code, but instead was hooked up to the app thread handling.

http_plugin.hpp add_async_api

Change from:

        void add_async_api(const api_description& api) {
           for (const auto& call : api)
              add_handler(call.first, call.second);
        }

To:

        void add_async_api(const api_description& api) {
           for (const auto& call : api)
              add_async_handler(call.first, call.second);
        }

Most importantly is to test this change as testing before obviously would not have actually tested the multi-threaded code is actually safe from multi-threads.

get_accounts_by_authorizers is the only HTTP endpoint using add_async_api. The trace api calls use add_async_handler directly.