Fix most llvm-12 clang-tidy warnings in IO code

Results can be seen here https://gitlab.cs.fau.de/aj46ezos/emper/-/jobs/385513.
The IoContext warning is legitim and should be fixed but it is solved by !201.

• Improve future member types
• GlobalIoContext fix type of cqe return type (s/uint32_t/int32_t/)
• Some returns types
• Ignore integer to pointer cast warning
• Fix SimpleDiskAndNetworkTest

Thanks llvm 12 :)