Handle initialisers/finalizer and Foreign.Concurrent

[iustin]

Hi,

Opening this issue to track an older bug report. See the thread Network.Curl cookie jar madness; in it, I found out that due to how handles are cleaned up (Foreign.Concurrent), they might or might now be actually cleaned up at any point during the program.

This is not a problem in general, but it prevents proper use of cookie jars across multiple handles.

A possible solution would be to provide a way for the user to force cleanup of a given handle.