This change reduces pressure on DB by doing the following things:
* not storing additional KV pair for SpentCoin
* storing Output right in the UnspentCoin, thus eliminating the need to get a
full transaction from DB
At the same time it makes UnspentCoin more fat and hot, but it should probably
worth it.
Also drop `GetUnspentCoinStateOrNew` as it shouldn't ever existed, UTXOs
can't come out of nowhere.
1.5M block import time (VerifyBlocks disabled) on AMD Ryzen 5 1600/16GB/HDD,
before:
real 302m9.895s
user 96m17.200s
sys 13m37.084s
after:
real 159m16.551s
user 69m58.279s
sys 7m34.334s
So it's almost two-fold which is a great improvement.
prevHash == input.PrevHash, so make less DB accesses and more real work. Fix
some bugs along the way:
* spentCoins structure may already be present in the DB when persisting TX,
there is nothing wrong with that and we shouldn't overwrite it
* it's only used for NEO and only to check for claim validity. Thus, when
processing claim tx the corresponding spentCoins should always be present
in the DB
It's useless work being done before it's actually needed. These (updated with
new values) are going to be written with some kind of Put anyway, so writing
them here is just a waste of time.
