Made the page cache discard read-ahead bufs until its evicter is initialized.

This discards read-ahead bufs this cache did not even ask for.
This is better than having a more complicated pre-initialization
and post-initialization state, which we would get if the balancer
was connected to the evicter before we set up read-ahead.
Made comments more informative.
Removed unused evicter_t::initialized method that I had just added.
Merge branch 'sam_2182' into v1.12.x
Merge branch 'v1.12.x' into next