Merge pull request #2267 from rethinkdb/atnnn/notes

Release notes for 1.12.3
Added page_t::is_deferred_loading and used it in an assertion in current_page_t::should_be_evicted.
Merge branch 'sam_2260' into v1.12.x
Make releasing block ids to the free list less fragile (removing make_non_deleted)
Keep current_page_ts alive for as long as there are any snapshots. Otherwise we would mess up the block versions.
Fixes a problem with current_page_acq_t destruction.
Fix unit test after change to block id release logic.
Draft release notes for 1.12.4