get_state_name(): return 'const string' instead of 'const char *'

Return a string instead of 'char *' to avoid usage of std::string:c_str()
to return a 'char *' from get_state_name() in mon/Monitor.h.

Related warning from cppcheck:
 [src/mon/Monitor.h:172]: (error) Dangerous usage of c_str(). The value
 returned by c_str() is invalid after this call.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
auth/Auth.h: prefer prefix ++operator for non-primitive types

Prefer prefix ++operator for non-primitive types like iterators for
performance reasons. Prefix ++/-- operators avoid creating a temporary
copy.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
CephxKeyServer.h: prefer prefix ++operator for iterators

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
SyntheticClient.h: prefer prefix ++operator for iterators

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>