Fix Nebulex.Adapters.Multilevel.get_all/3 to call the same invoked function into the underlying levels

[cabol]

Currently, when calling Nebulex.Adapters.Multilevel.get_all/3, it is translated into multiple get/3 operations underneath, instead of calling the same invoked function for all the levels. This causes a significant performance penalty, avoiding the underlying levels to resolve the get_all themselves.

For example, the Redis adapter is able to run the get_all in a single command because it supports the MGET operation, but when using the multi-level adapter it is not being called since the adapter converts the get_all into multiple gets before calling the underlying adapters.