blockchain: better Ancestor with skiplists

[kcalvinalvin]

On startup, Ancestor call was taking a lot of time when the node was loading the blockindex onto memory. This change speeds up the Ancestor function significantly and speeds up the node during startup.

On testnet3 at blockheight ~2,500,000, the startup was around 30seconds on current main and was 5 seconds with this change. Below is a benchstat result showing the significant speedup.

goos: darwin
goarch: arm64
pkg: github.com/utreexo/utreexod/blockchain
           │     old.txt      │               new.txt                │
           │      sec/op      │    sec/op     vs base                │
Ancestor-8   120819.301µ ± 5%   7.013µ ± 19%  -99.99% (p=0.000 n=10)

           │  old.txt   │            new.txt             │
           │    B/op    │    B/op     vs base            │
Ancestor-8   0.000 ± 0%   0.000 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal

           │  old.txt   │            new.txt             │
           │ allocs/op  │ allocs/op   vs base            │
Ancestor-8   0.000 ± 0%   0.000 ± 0%  ~ (p=1.000 n=10) ¹
¹ all samples are equal