entity-manager: remove global io_context

Remove the global var
```
boost::asio::io_context io
```
and move it to be a local var in the main function.

Since boost::asio::io_context io is declared first, it in scope for the
entire program duration and should not cause any issues from that
perspective.

The io_context is passed through where needed. In case there is a class
already defined, the class now has a reference to the io_context to
avoid passing it through everywhere.

Tested: Capturing or passing a reference which is always valid should
not introduce any issues.

Tested on Tyan S8030:

```
Jul 01 09:59:26 s8030-bmc-30303035c0c1 entity-manager[4204]: Inventory Added: Supermicro PWS 920P SQ 0
Jul 01 09:59:26 s8030-bmc-30303035c0c1 entity-manager[4204]: Inventory Added: Supermicro PWS 920P SQ 1
Jul 01 09:59:26 s8030-bmc-30303035c0c1 entity-manager[4204]: Inventory Added: chassis
Jul 01 09:59:26 s8030-bmc-30303035c0c1 entity-manager[4204]: Inventory Added: MBX 1.57 Chassis
```

busctl tree output as before

Change-Id: Ie8f7d18c38d166c57a9cb645ab45c9103bbdff6e
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>
diff --git a/src/entity_manager/perform_scan.hpp b/src/entity_manager/perform_scan.hpp
index 54d0e67..17396d4 100644
--- a/src/entity_manager/perform_scan.hpp
+++ b/src/entity_manager/perform_scan.hpp
@@ -27,7 +27,7 @@
 {
     PerformScan(EntityManager& em, nlohmann::json& missingConfigurations,
                 std::list<nlohmann::json>& configurations,
-                std::function<void()>&& callback);
+                boost::asio::io_context& io, std::function<void()>&& callback);
 
     void updateSystemConfiguration(const nlohmann::json& recordRef,
                                    const std::string& probeName,
@@ -41,6 +41,8 @@
     bool _passed = false;
     MapperGetSubTreeResponse dbusProbeObjects;
     std::vector<std::string> passedProbes;
+
+    boost::asio::io_context& io;
 };
 
 } // namespace scan