[mono-threads] Fix lifetime of MonoThreadInfo->handle (#3539)

[mono-threads] Fix lifetime of MonoThreadInfo->handle (#3539)

This handle should have a total of maximum 3 copies:
 - in MonoThreadInfo->handle
 - in the return value of mono_threads_create_thread
 - in MonoInternalThread->handle (if it's a managed thread)

Before, there would always be 2 references (in MonoThreadInfo->handle and another another for MonoInternalThread->handle), even if it would be referenced at one place only (MonoThreadInfo->handle), which would lead to a handle leak.

We also need to duplicate the returned handle, to avoid having a use-after-free of the handle (in case the thread exits before we access that handle)