pAcommsHandler can took too long to initialize modems and MOOSDB mail gets dropped

[tsaubergine]

With multiple drivers, pAcommsHandler may not read the mail often enough while running ModemDriverBase::startup() and thus the CMOOSApp gives the following warning:

Too many unread incoming messages [2055] : purging

Thus, data are lost at this startup phase. We need to run the ModemDriverBase::startup() in a separate thread, or some other such mitigation method to continue reading mail.