Fix warnings introduced by mCBM MRs + some similar ones

• Fix mismatch in old MQ samplers with current flesnet IPC, removing warnings
• Fix new sign warnings introduced by mCBM MRs
• Fix deprecation warning for ROOT serdes method in old Histo Server MQ device
• Fix unused variable warnings introduced by mCBM MRs

Probably complementary of !863 (closed) (but should be fixing only new warnings)