media_cache: Don't lock when curl the remote file

When playing a remote sound file, which is not in cache, first we need
to download it with ast_bucket_file_retrieve.

This can take a while if the remote host is slow. The current CURL
timeout is 180secs, so in extreme situations, it can take 3 minutes to
return.

Because ast_media_cache_retrieve has a lock on all function, while we
are waiting for the delayed download, Asterisk is not able to play any
more files, even the files already cached locally.

ASTERISK-29544 #close

Change-Id: I8d4142b463ae4a1d4c41bff2bf63324821567408