Bug 4238: assertion Read.cc:205: "params.data == data"

Bug 4238: assertion Read.cc:205: "params.data == data"

Inside IdleConnList::findUseable the IdleConnList::removeAt call can delete
"this" IdleConnList object. The IdleConnList::clearHandlers called imediatelly
after the removeAt method, will try to use the invalid "this" object in
a comm_read_cancel function call, causing this assertion or other similar.

This patch fixes the IdleConnList::findUseable, IdleConnList::pop and
IdleConnList::findAndClose methods to call IdleConnList::clearHandlers before
the IdleConnList::removeAt is called.

This is a Measurement Factory project