Add LoadExternalBlockFile() benchmark
Summary:
For future reference:
| ns/op | op/s | err% | total | benchmark |--------------------:|--------------------:|--------:|----------:|:---------- | 169,155,403.00 | 5.91 | 0.6% | 1.88 | `LoadExternalBlockFile`
This is a partial backport of core#16981
https://github.com/bitcoin/bitcoin/pull/16981/commits/48a68908ba3d5e077cda7bd1e908b923fbead824
Test Plan: ninja bench-bitcoin
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D15959