refactor: replace RecursiveMutex `cs_vProcessMsg` with Mutex (and rename)

This PR is related to #19303 and gets rid of the RecursiveMutex cs_vProcessMsg. Both of the critical sections only directly access the guarded elements, i.e. it is not possible that within one section another one is called, and we can use a regular Mutex: https://github.com/bitcoin/bitcoin/blob/e3ce019667fba2ec50a59814a26566fb67fa9125/src/net.cpp#L1597-L1602 https://github.com/bitcoin/bitcoin/blob/e3ce019667fba2ec50a59814a26566fb67fa9125/src/net_processing.cpp#L4142-L4150 Also, it is renamed to adapt to the (unwritten) naming convention to use the _mutex suffix rather than the cs_ prefix.