Make eleveldb a depdendency of the mnesia_eleveldb app

[sargun]

[richcarl]

Hi Sargun! It's probably a good idea, but so as to ensure that users can start mnesia without having to manually start eleveldb first, please also change the call to application:start() in mnesia_eleveldb:init_backend() to use application:ensure_all_started() instead.

[v-kat]

I wanted to try using erocksdb which should also be compatible, instead, so I kinda prefer the existing behavior.

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