Remove `default_primary_key_type`  and extract contains of `native_database_types` to a constant since they aren't conditional now in SQLite3Adapter. Makes it more like other adapters.
Merge pull request #12527 from vipulnsward/sqlite_database_types

SQLite3 Adapter datatypes cleanup
`$SAFE = 4;` has been removed with Ruby 2.1

For background -
https://bugs.ruby-lang.org/issues/8468
Changset - https://bugs.ruby-lang.org/projects/ruby-trunk/repository/revisions/41259/diff/test/ruby/test_thread.rb
Merge pull request #12530 from vipulnsward/remove_thread_safe

`$SAFE = 4;` has been removed with Ruby 2.1