maestro/pool.h does not compile with C++ because the keyword 'namespace' is used

Note that extern "C" is about linkage and does not guard against keywords appearing in a header file. Here is a list of current C++ keywords: https://en.cppreference.com/w/cpp/keyword.

The fix is trivial; there just needs to be an agreement of what the new name should be, especially as it also appears in documentation.

Branch: 'subscriptions'