Allowed unloading of a single module.

unloadAll() forces destruction of Owned pointers to the dynamic
library handle. This implicitly causes a dlclose() of all the loaded
module libraries. Unloading a dynamic library can have side effects if
some shared state (e.g., a module) in Mesos points into the library.

Futher more, removed the existing unloadAll() that is no longer
needed.

Review: https://reviews.apache.org/r/26789