mgr: use new Gil class in place of PyGILState_*() API

mgr: use new Gil class in place of PyGILState_*() API

Prep work for loading modules in separate python sub-interpreters.
According to the Python C API docs, mixing the PyGILState_*() API
with sub-interpreters is unsupported, so I've replaced these calls
with a Gil class, which will acquire and release the GIL against
a specific python thread state.

Note the manual python thread state creation in MgrPyModule::serve().
The PyGILState_*() APIs would have done that for us, but we're not
using them anymore, so have to do it by hand (see
https://docs.python.org/2/c-api/init.html#non-python-created-threads
for some description of this).

Signed-off-by: Tim Serong <tserong@suse.com>