Resolve "Support other KV Databases"

Closes #173 (closed)

This MR adds support for different metadata backends. Actually, PARALLAX from FORTH is supported as experimental. PARALLAX needs a 8GB file (minimum size) to store metadata. It is created in the metadir directory as rocksdbx name. Inside the install script there is a commented line, to modify an internal parameter of PARALLAX increasing the key size drom 255 bytes to 4096 (Max PATH name in POSIX). However, PARALLAX is not (fully) tested with such values.

This backend should work in all the scenarios, but there are some issues that appear under high load mdtest.

This MR includes the optional compilation/dependencies of each of the backends. Testing is generated dynamically from conftest.template depending on the options selected.

This MR includes a new argument for the daemon to remove the rootdir/metadir files directories when the daemon is finished. This reduces the data from artifacts when running databases that uses a fixed size file. (i.e., Parallax)