Various additions to bench driver + fprof header for search

Made various changes to help facilitate benchmarking and performance hunting.

* Added a `yz-fprof: <filename>` header to the search resource.  If this header is present the search request will be traced with fprof and the file `<filename>.analysis` will be written to that nodes CWD.

* Added a random fruit search operation to the bench driver.  This allows to run random conjunection queries against fruit data-load.  The max number of terms and max cardinality of a term can be bound.  A field list `FL` may also be passed.

```erlang
{random_fruit_search, FL, MaxTerms, MaxCardinality}
```

* Fixed some bugs with the mfa/cache server (the poorly named `yz_file_terms`).  I'm going to be using this server more to benchmark Yokozuna against real data.