* New data in "Summary" on Block pages (supported for bitcoind v0.17.0+)
* Fee percentiles
* Min / Max fees
* Input / Output counts
* Outputs total value
* UTXO count change
* Min / Max tx sizes
* Start of RPC API versioning support
- support for configurable RPC concurrency level (default 10, to be under bitcoind default "rpcworkqueue=16")
- queue up requests to prevent overloading RPC work queue of bitcoind
- simplify / cleanup rpcApi
- updated package-lock
- include sample .env file for easy reference
- continue to support key-by-key updating from credentials.js for now
- minor code formatting
- reflect status of BTCEXP_NO_RATES in UI when applicable (hide new exchanged-currency items from display dropdown and hide exchange rate on homepage)
- rename _LOGIN to _BASIC_AUTH_PASSWORD for clarity
- tweak README to call out .env-sample
- cleaner support for handling credentials (new app/defaultCredentials.js has the defaults, git-ignored app/credentials.js overwrites)
- update some dependencies