lavc: replace and deprecate the lock manager

Use static mutexes instead of requiring a lock manager. The behavior
should be roughly the same before and after this change for API users
which did not set the lock manager at all (except that a minor memory
leak disappears).

doc/APIchanges

@@ -15,6 +15,11 @@ libavutil:     2017-10-21
 
 API changes, most recent first:
 
+2017-xx-xx - xxxxxxx - lavc 58.9.100 - avcodec.h
+  Deprecate av_lockmgr_register(). You need to build FFmpeg with threading
+  support enabled to get basic thread-safety (which is the default build
+  configuration).
+
 2017-12-xx - xxxxxxx - lavu 56.7.100 - cpu.h
   AVX-512 flags added.