curlbasesink: Rename curl transfer thread

Some systems restrict the length of thread names to 16 bytes.

gstreamer/gst-plugins-bad#838
4 jobs for curlsink-thread-name-fix in 7 minutes and 14 seconds (queued for 35 seconds)
Status Job ID Name Coverage
  Preparation
passed #57860
gst indent

00:00:29

passed #57859
manifest

00:00:24

 
  Build
passed #57861
build fedora x86_64

00:02:45

 
  Full Builds
passed #57862
build android arm64 api28

00:03:34