[wp] using sub-second timeout and 2s default

src/libraries/utils/task.ml

src/libraries/utils/task.mli

src/plugins/wp/Cache.ml

src/plugins/wp/Cache.mli

src/plugins/wp/ProverTask.ml

src/plugins/wp/ProverTask.mli

src/plugins/wp/ProverWhy3.ml

src/plugins/wp/ProverWhy3.mli

src/plugins/wp/VCS.ml

src/plugins/wp/VCS.mli

src/plugins/wp/doc/manual/wp_plugin.tex

src/plugins/wp/gui/GuiProver.ml

src/plugins/wp/register.ml

src/plugins/wp/wp_parameters.ml