```
Add benchmarks for mempool acceptance of chained transactions. This adds tests for:
- 50 tx sized 1-input-1-output chain
- 500 tx sized 1-input-1-output chain
- 63 tx sized 2-inputs-1-output tree
- 511 tx sized 2-inputs-1-output tree
```
Port of bchn#1071.
The code is adapted for the new nanobench framework.