taskpool: expose new "shared" task pool implementation

taskpool: expose new "shared" task pool implementation

While the default implementation will spawn a thread per new
pushed task, this new implementation instead spawns a maximum
number of threads, then queues new tasks on existing threads.

The thread that the new task will be queued on is picked in
a pretty naive fashion, by simply popping the first thread
from a queue and pushing it back to the tail, but this is
an implementation detail and can always be sophisticated
in the future if the need arises.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/692>