Add getBlockTime rpc api

[CriesofCarrots]

Problem

It's strongly desired by some users to have a network-controlled best effort timestamp associated with the production time of each block.
Solana does not offer any RPC apis that return this information.

Summary of Changes

• Adds getBlockTime method to rpc
• Plumbs back to blocktree; returns dummy results, where the last median time is always timestamp 0

Toward #7115 (Steps 1 & part of 2)

[mvines]

Thanks, plumbing looks great. Sneak in a cli/ command for this too? solana get-block-time <SLOT>?

[codecov]

Codecov Report

Merging #7130 into master will decrease coverage by 2.2%.
The diff coverage is 60%.

@@           Coverage Diff            @@
##           master   #7130     +/-   ##
========================================
- Coverage      72%   69.8%   -2.3%     
========================================
  Files         230     230             
  Lines       49393   51123   +1730     
========================================
+ Hits        35576   35697    +121     
- Misses      13817   15426   +1609

Pull request has been modified.

[CriesofCarrots]

@mvines , do you mind taking a look at the changes I made to PohConfig::default and genesis::main?
There was some really unfortunate rounding due to use of Duration::from_millis() on our particular clock defaults (which meant I could never get slot_duration = 400ms from slots_per_year). Since we are stashing a Duration anyway, I saw no reason not to use Duration::from_micros().

[mvines]

There was some really unfortunate rounding due to use of Duration::from_millis() on our particular clock defaults (which meant I could never get slot_duration = 400ms from slots_per_year).

What duration where you getting? Just curious

Since we are stashing a Duration anyway, I saw no reason not to use Duration::from_micros().

It feels a little weird to be using such a small unit (micro seconds) on the solana-genesis command-line, wdyt about keeping --target-tick-duration as ms and converting to μs internally

[CriesofCarrots]

What duration where you getting? Just curious

384ms (1000/64 rounds to 6, instead of 6.25)

It feels a little weird to be using such a small unit (micro seconds) on the solana-genesis command-line, wdyt about keeping --target-tick-duration as ms and converting to μs internally

👌