When serializing multiple accounts, cost of a buffer grow
can become significant. This commit tries to amortize it by
reusing the same buffer in a single `Persist()` call.
1.5M block import time (VerifyBlocks disabled) on AMD Ryzen 5 1600/16GB/HDD,
before:
real 159m16.551s
user 69m58.279s
sys 7m34.334s
after:
real 139m41.836s
user 67m12.477s
sys 6m19.420s
12% which is even a bit more than could be expected from inputs analysis (that
has around 10% cache hits for a block-wide cache), worth doing.
Importing 100K blocks starting at 1.4M, before this patch:
real 6m0,356s
user 8m52,293s
sys 0m47,372s
After this patch:
real 4m17,748s
user 6m23,316s
sys 0m37,866s
Almost 30% better.
