lib/cache: abort() if emergency cache-clear fails

As the code has been so far, there's no usable cache in that case
and some code just can't handle that.  Up to now we were getting
SIGSEGV from inside LMDB on the next attempted operation.

We might consider loosening preallocation in that case or even
retrying after a short sleep.  Systemd's restart after hold-off
timeout has an effect similar to the short sleep.