Fix timestamp overflow

[CriesofCarrots]

Problem

On a testnet with very high stakes, solana get-block-time returns unexpected, unrealistic value (see #7781 ).
Stake-weighted blocktime calculation accumulators currently use u64 integer types; stake quantities are represented in lamports, and also use the u64 integer type. Thus, it is easy to make the stake-weighted timestamp calculation overflow when multiplying a moderately high stake number with a UnixTimestamp (i64).

(This bug produces a skewed timestamp on testnet, instead of panicking the node as it does in the unit test; TIL Rust only checks for integer overflow in debug builds.)

Summary of Changes

• Use u128 integers to handle stake-weighting timestamps
• Also sneak in fix to Vote error occurring frequently in slot 1

Fixes #7781

[codecov]

Codecov Report

Merging #7886 into master will increase coverage by <.1%.
The diff coverage is 97.8%.

@@           Coverage Diff            @@
##           master   #7886     +/-   ##
========================================
+ Coverage    81.8%   81.9%   +<.1%     
========================================
  Files         244     244             
  Lines       51902   51983     +81     
========================================
+ Hits        42506   42585     +79     
- Misses       9396    9398      +2

[ryoqun]

@CriesofCarrots Thanks for fixing this!!