Fix v8 dependency issues in the ubuntu package

Closes #2101
Closes #2261
OTS review by @Tryneus
Release notes for 1.12.3
Second draft
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
Merge branch 'v1.12.x' into next