syz-manager: re-enable corpus rotation
syz-manager: re-enable corpus rotation

Let's give it another chance since the previous coverage drop wasn't
completely conclusive.
Few changes that may help:
 - do rotation at 1/5 instead of 1/3 rate
 - don't send triage candidates to rotated VMs,
   since they may not have required syscalls enabled
 - don't send new inputs/coverage to rotated VMs,
   let them run in complete isolation

Update #1348

xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller