Make bio.c threads killable ASAP if needed.

We use this new bio.c feature in order to stop our I/O threads if there
is a memory test to do on crash. In this case we don't want anything
else than the main thread to run, otherwise the other threads may mess
with the heap and the memory test will report a false positive.

src/bio.h

@@ -33,6 +33,7 @@ void bioCreateBackgroundJob(int type, void *arg1, void *arg2, void *arg3);
 unsigned long long bioPendingJobsOfType(int type);
 void bioWaitPendingJobsLE(int type, unsigned long long num);
 time_t bioOlderJobOfType(int type);
+void bioKillThreads(void);
 
 /* Background job opcodes */
 #define REDIS_BIO_CLOSE_FILE    0 /* Deferred close(2) syscall. */