Add Initialize/Terminate method to Platform base plugin

Platform holds a smart pointer to each platform object created in a
static variable what cause the platform destructors called only on
program exit when other static variables are not availables. With this
change the destructors are called on lldb_private::Terminate()

+ Fix DebuggerRefCount handling in ScriptInterpreterPython

Differential Revision: http://reviews.llvm.org/D7590

llvm-svn: 228944