Added statistic logs about database state in the end of benchmark

[VladGan]

What is the goal of this PR?

We added statistic logging to let the user know how many elements are there in the database at the end of the benchmark run.

What are the changes implemented in this PR?

• added statistic queries: 'thing', 'attribute', 'relation' and 'entity' counts for grakn, 'node' and 'relationship' counts for neo4j;
• added duration log for statistic calculation itself.