Reflection.Emit is not ThreadSafe

Users are not supposed to operate on same Reflection.Emit object on multiple threads. Reflection.Emit Objects are *NOT* thread-safe. It is certainly OK to operate on different Reflection.Emit objects on different threads.

We have been discovering issues when users trying to do the first scenario and we are trying hard to make the program not to crash. :) But the correctness cannot be guaranteed. So be careful.