perf: pass around a cached block hash during block validation

[PastaPastaPasta]

this change saw a ~38% performance improvement in header sync reindex

reproduce via time ./src/qt/dash-qt --nowallet --testnet --reindex --stopatheight=5

On Develop this took average of 1:48 to finish, on this branch it took 1:07

Issue being fixed or feature implemented

Slow header / block validation

What was done?

Pass around cached block hash

How Has This Been Tested?

Reindexed testnet

Breaking Changes

None

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation
• I have assigned this pull request to a milestone (for repository code-owners and collaborators only)

[UdjinM6]

118.04s user 10.80s system 113% cpu 1:53.43 total
vs
96.26s user 9.98s system 113% cpu 1:33.74 total
in my case

build is broken btw, c284831 should help

also, I'm getting comparable results
96.47s user 9.34s system 111% cpu 1:35.26 total
with a much simpler diff 40a34f2

[PastaPastaPasta]

I think I like your diff; applied

[UdjinM6]

ACK

[ogabrielides]

ACK