Cli: add cluster-date subcommand, and make block-time slot optional

[CriesofCarrots]

Problem

The cluster calculates the date as a function of slots from genesis, and it uses that date to determine if a stake account is locked. If we want to set an unlock for something like "one day from now", there's no RPC endpoint to ask the cluster what it thinks "now" is.

Summary of Changes

• Add solana cluster-date command to return sysvar::Clock.unix_timestamp from the most recent cluster-confirmed root
• Make solana block-time slot param optional, and return getBlockTime from the most recent cluster-confirmed root if None
• --output json available for both commands

$ solana cluster-date --url http://api.mainnet-beta.solana.com

Block: 9198022
Date: 2020-04-28T04:29:09Z (UnixTimestamp: 1588048149)

$ solana block-time --url http://api.mainnet-beta.solana.com

Block: 9198023
Date: 2020-05-05T00:22:27Z (UnixTimestamp: 1588638147)

Oof, almost a week difference (cc #9874 )

Fixes #9847

[garious]

I don't understand the difference between solana block-time with no slot and solana cluster-date? Also, strange format with Block: on one line and nothing like Date: on the other. And where tests?

[CriesofCarrots]

I don't understand the difference between solana block-time with no slot and solana cluster-date?

Well, solana block-time calls the getBlockTime rpc, which queries the validator timestamp oracle.
solana cluster-date queries a Bank for its sysvar::Clock.unix_timestamp

I am not clear on why the validator timestamp oracle was not appropriate for lockup timestamp needs, but we do have 2 sources of timestamps, and @mvines suggested the solana block-time with no slot to exemplify this fact as well as demonstrate the drift between them. Ref: #9847 (comment)

[garious]

Got it, thanks. LGTM

[codecov]

Codecov Report

Merging #9878 into master will decrease coverage by 0.0%.
The diff coverage is 27.4%.

@@           Coverage Diff            @@
##           master   #9878     +/-   ##
========================================
- Coverage    80.4%   80.4%   -0.1%     
========================================
  Files         283     283             
  Lines       64936   64977     +41     
========================================
- Hits        52245   52242      -3     
- Misses      12691   12735     +44