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.