dbus_singleton: use stack variable and extern

Currently, the |systemBus| connection is a static variable declared in
headers. This has a problem that every translation unit will keep its
own copy. It's not a problem today because there's only one translation
unit "webserver_main.cpp.o". This issue was brounght up in
https://gerrit.openbmc.org/c/openbmc/bmcweb/+/54758

Actually, the |systemBus| doesn't need to be a singleton. It can just be
a stack variable, which is normally more efficient than heap variables.

To keep minimum changes treeside, this commits keeps the existing
|systemBus| variable as an external variable. It is defined in its own
translation unit. It is initialized in the main translation unit.

Reference:
1. Extern
https://stackoverflow.com/questions/1433204/how-do-i-use-extern-to-share-variables-between-source-files



Tested:
1. Romulus QEMU robot Redfish test passed;
2. Start and restart service on real hardware, no issues;
3. No new validator failures
4. Code compies

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: I03b387bd5f218a86c9d1765415a46e3c2ad83ff9