trap-manager: Wait for install to finish before uninstalling

trap-manager: Wait for install to finish before uninstalling

There was a race condition between install() and uninstall()
where one thread was in the process of installing a trap
entry, and had destroyed the child_sa, while the other
thread was uninstalling the same trap entry and ended up
trying to destroy the already destroyed child_sa, resulting
in a segmentation fault in the destroy_entry() function.

The uninstall() function needs to wait until all the threads
are done with the installing before proceeding to uninstall
a trap entry.

Closes strongswan/strongswan#131.