It's more generic and convenient than MillisecondsPerBlock. This setting is
made in backwards-compatible fashion, but it'll override SecondsPerBlock if
both are used. Configurations are specifically not changed here, it's
important to check compatibility.
Fixes#2675.
HF_ prefix makes zero sense to me. If it's "hardfork", then it's in the
"Hardforks" section already. If it's "hotfix", then it made some sense back
when it was HF_2712_FixSyscallFees, but now it's codenamed anyway. So we can
drop it and have a cleaner config.
