fix(tarantool-sys): fix graceful stopping

Was broken because tarantool_free checks if the current process is the main one and not the child, which was forked at some point (at what point?). This check was implemented by saving the original process's id in the static variable master_pid, which got set when the code got loaded the first time into memory.

So we broke this when we started forking the process in picodata, which resulted in master_pid being set to the pid of the picodata's "supervisor" process, which doesn't even enter tarantool runtime.

Closes #37 (closed)