margo, argobots and mercury have been updated. The abt-snoozer dep
required by margo it is not needed anymore.

RocksDB has been also updated to the lastest release

Instead of using custom variable to search for installed libraries
we can rely on the standard `CMAKE_PREFIX_PATH` [1]

[1]: https://cmake.org/cmake/help/v3.8/variable/CMAKE_PREFIX_PATH.html#variable:CMAKE_PREFIX_PATH

Avoid the usage of a custom cmake "find-module" for mercury.
Instead use the official exported target

gflags is needed to compile rocksDB but not to link with it

Setting the properly the JMALLOC_LIBRARIES variable into the find module
made possible to avoid the conditonal inclusion of the library

Previously, we gave already installed libfabric versions no chance
to be detected over exclusively installed libfabric

When rocksdb is build with jemalloc linking with adafs was failing