Summary:
To measure the results of upcoming DeleteRange v2 work, this commit adds
simple benchmarks for RangeDelAggregator. It measures the average time
for AddTombstones and ShouldDelete calls.

Using this to compare the results before #4014 and on the latest master (using the default arguments) produces the following results:

Before #4014:
```
=======================
Results:
=======================
AddTombstones:          1356.28 us
ShouldDelete:           0.401732 us
```

Latest master:
```
=======================
Results:
=======================
AddTombstones:          740.82 us
ShouldDelete:           0.383271 us
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4363

Differential Revision: D9881676

Pulled By: abhimadan

fbshipit-source-id: 793e7d61aa4b9d47eb917bbcc03f08695b5e5442