SmrSession: init $db only once
SmrSession: init $db only once

Because SmrSession::init() is always called any time this class is
loaded (done right after the class definition), we can get away with
only initializing the `SmrMySqlDatabase` once, in `init`.

hemberger/smr,hemberger/smr,smrealms/smr,smrealms/smr,hemberger/smr,smrealms/smr,hemberger/smr