This fixes a sporadic failure on a 1/2 core VM where
"git cat-file --batch" hasn't started up by the time
$cleanup->() destroys the ALL.git directory in t/lei.t
(but not t/lei-oneshot.t).
This happens because dwaitpid() runs inside the event loop
asynchronously and we were able to return to the client before
the cat-file process could even start.
I could not reproduce this failure on my usual 4-core
workstation via "schedtool -a 0x1" to force the entire
test to use a single core.
Lazy transactions matches OverIdx and SearchIdx behavior, and
I've verified this lets us avoid problems with old Xapian
versions (on CentOS 7.x) which failed to set FD_CLOEXEC.