Med: libfencing: minimize slowdown when fetching metadata

Commit 12cf7b9 limited bad impact of never-finishing fence agent when
asked for metadata, but imposed very impractical penalty of 1 second
unless the scheduler went crazy and favored the child being waited
for against the parent that waits for it to finish (as normally, first
its non-blocking wait-probe was too early). This commit splits 1 s
sleeps into 100 ms ones, lowering the delay by an order of magnitude.
Alternatively, there could be a more convoluted arrangement using
eventloop, timer and SIGCHLD handling, but it would come at its own
costs.