Fix issues with testnet dir, update docs (#992)

* Fix issues with testnet dir, update docs

* Remove "simple testnet" docs

* Tear out old "bn testnet" stuff

* Add back ClientGenesis::Interop

* Tidy

* Remove lighthouse-bootstrap module

* Fix bug with spec constant mismatch

* Ensure beacon-node.toml is written to correct dir

* Add -t alias for --testnet-dir

* Update book/src/local-testnets.md

Co-Authored-By: Age Manning <Age@AgeManning.com>

* Add --purge CLI flag

* Update purge docs

* Perform manual delete of files in purge

* Rename --purge to --purge-db

* Address Michael's comments

Co-authored-by: Age Manning <Age@AgeManning.com>

beacon_node/beacon_chain/Cargo.toml

@@ -15,7 +15,6 @@ store = { path = "../store" }
 parking_lot = "0.9.0"
 lazy_static = "1.4.0"
 lighthouse_metrics = { path = "../../eth2/utils/lighthouse_metrics" }
-lighthouse_bootstrap = { path = "../../eth2/utils/lighthouse_bootstrap" }
 log = "0.4.8"
 operation_pool = { path = "../../eth2/operation_pool" }
 rayon = "1.2.0"