A bug by static variables in RunMainLoop() 

Dear contributors,

RunMainLoop() in mtcp/src/core.c:761 is supposed to be executed per core, but has static variables, "static uint16_t len, static uint8_t *pktbuf" at line 787 and 788 thereby being not thread-safe and occurring unexpected problems.
I think just removing the keyword "static" will be okay. Please consider any changes to fix the problem.
Thank you