benchmarks/blogbench: fix rand()-related crash

blogbench uses rand(3) unsafely from different threads which leads to crashes.
Use arc4random(3) instead.

Bump PORTREVISION

While here, relocate USES variable.

PR: 248420
Submitted by: sigsys@gmail.com
Approved by: jmohacsi@bsd.hu (maintainer)
MFH: 2020Q3 (runtime fix)