Add repeatable NodeStore timing benchmark:
The timing test is changed to overcome possible file buffer cache effects by creating different read access patterns. The unittest-arg command line arguments allow running the benchmarks against any of the available backends and altering the parameters passed in the same format as rippled.cfg. The num_objects parameter permits variation of the number of key/values inserted. The data is random but matches reasonably well the values that rippled might generate.
Add new RocksDBQuickFactory for benchmarking:
This new factory is intended for benchmarking against the existing RocksDBFactory and has the following differences.
* Does not use BatchWriter
* Disables WAL for writes to memtable
* Uses a hash index in blocks
* Uses RocksDB OptimizeFor… functions
See Benchmarks.md for further discussion of some of the issues raised by investigation of RocksDB performance.