Let profile() show backend accesses within query optimization

#2189 introduced an optimization mechanism, which utilizes backend accesses. However, there is currently no way to show those when executing a query with `profile()`. But because profiling a query should reveal information about the complete execution process, the optimization phase should also be included.

the link to the [original discussion](https://github.com/JanusGraph/janusgraph/pull/2189#discussion_r501668591) provides more details and ideas for a solution.

Start by reading the implementation of profile() and the optimization mechanism introduced in #2189, then review the linked discussion for the proposed approach. The work is complete when profiling a query includes backend accesses from the optimization phase and tests verify the expanded profile output.

Written by the indexing model from the issue text.