In theory, one can do std::mem::forget, and ignore the drop handler. With
the lifetime hack, this could result in a crash.
So we simply require 'static lifetime now (futures also needs that).
Fix a potential bug where errors that happen after the SendHandle has
been dropped while doing the thread join might have been ignored.
Requires internal check_abort to be moved out of 'impl SendHandle' since
we only have the Mutex left, not the SendHandle.
Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
