Add a simple fabric benchmark

This is mostly a diagnostic tool in the spirit of couch_debug. It creates a
database, fills it with some docs, and then tries to read them. It computes
rough expected rates for doc operations: how many docs per second it could
insert, read, get via _all_docs, etc. When the test is done, it deletes the
database. If it crashes, it also deletes the database. If someone brutally
kills it, the subsequent runs will still find old databases and delete them.

To run a benchmark:
```
  fabric_bench:go().
```

Pass parameters as a map:
```
  fabric_bench:go(#{doc_size=>large, docs=>25000}).
```

To get available options:
```
  fabric_bench:opts()
```

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