Improved the sindex_wait() polling nap logic. Minor cleanup of the sindex not ready error handling.
Fixed a bug in the sindex status retrieval. It only considered the first shard multiple times, rather than considering all shards.
adding --hard option to rethinkdb-import
Merge branch 'next' into daniel_2179
adding --hard option to rethinkdb-restore
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
Merge branch 'daniel_2179' into next
changing default cache size to be smaller, but adding a minimum size of 100 MB, extra warnings about low cache size or low available memory
changing --hard to --hard-durability in import/restore
Merge branch 'grey_issue_2175' into next
merging with next
changing disk-backed queues to have a base cache size of 2 MB