Limited to 1 the duration-discovery threads number

This should fix an occasional deadlock (apparently on the GIL) when creating media-cues the first time.
Unfortunately this slows a bit the process of retrieving files duration.

lisp/backends/gst/gst_media.py

@@ -19,7 +19,7 @@
 
 import weakref
 from concurrent.futures import ThreadPoolExecutor
-from os import path, cpu_count as _cpu_count
+from os import path
 
 from lisp.backends.base.media import Media, MediaState
 from lisp.backends.gst import elements
@@ -29,10 +29,6 @@
 from lisp.core.has_properties import Property
 
 
-def cpu_count():
-    return _cpu_count() if _cpu_count() is not None else 1
-
-
 def validate_pipeline(pipe):
     # The first must be an input element
     if pipe[0] not in elements.inputs().keys():
@@ -316,7 +312,7 @@ def __on_eos(self):
             self.eos.emit(self)
             self.interrupt(emit=False)
 
-    @async_in_pool(pool=ThreadPoolExecutor(cpu_count()))
+    @async_in_pool(pool=ThreadPoolExecutor(1))
     def __duration(self):
         self.duration = gst_uri_duration(self.input_uri())